In 1796, English physician Edward Jenner noticed something that dairymaids in Gloucestershire seemed to understand intuitively: women who had contracted cowpox — a mild disease caught from infected cattle — appeared to be immune to smallpox, one of the most lethal diseases in human history. Jenner tested this observation by taking material from a cowpox sore on dairymaid Sarah Nelmes and inoculating eight-year-old James Phipps with it. He then exposed the boy to smallpox material. Phipps did not develop smallpox.
Jenner had not discovered a vaccine in the molecular sense — he had no idea what a virus was, what an immune system did, or how immunity worked. But he had identified the practical principle that would eventually save more human lives than any other medical intervention: exposing the immune system to a harmless version of a pathogen prepares it to defeat the real thing.
That principle, refined over two centuries of immunology, virology, molecular biology, and clinical medicine, is how vaccines work. The details are now understood in remarkable depth — from the molecular interactions between antigen and antibody to the genetic instructions of mRNA vaccines. Vaccines remain the most cost-effective public health intervention ever developed.
"The development of vaccines has been one of the greatest triumphs of modern medicine. In the 20th century alone, vaccines eliminated or dramatically reduced diseases that had been feared throughout human history." — World Health Organization, State of the World's Vaccines and Immunization (2009)
The scale of the achievement is difficult to overstate. The WHO estimates that between 2000 and 2019, vaccines prevented over 37 million deaths from measles alone (Santoli et al., 2020). Smallpox, which killed an estimated 300 million people in the 20th century before eradication efforts began, was declared eliminated from the planet in 1980 — the only human infectious disease ever eradicated. Polio, which paralyzed hundreds of thousands of children annually in the 1950s, now exists in only a handful of countries. These outcomes are not the product of improved nutrition or sanitation alone; they are the direct result of mass vaccination campaigns.
Key Definitions
Vaccine — A biological preparation that provides acquired immunity to a specific disease by exposing the immune system to an antigen — a harmless version or component of a pathogen — without causing the disease. Triggers an immune response and the formation of memory cells that enable rapid, strong response upon subsequent exposure to the real pathogen.
Antigen — Any substance recognized by the immune system as foreign, triggering an immune response. For pathogens, antigens are typically surface proteins (like the spike protein of SARS-CoV-2 or the hemagglutinin of influenza). Vaccines introduce specific antigens to train the immune system without delivering the whole, dangerous pathogen.
Antibody — A protein produced by B cells (B lymphocytes) that binds specifically to a particular antigen. Antibodies neutralize pathogens by blocking their ability to infect cells, marking them for destruction by immune cells (opsonization), or activating the complement system. The specificity of antibodies — each binds to a specific antigen — is the foundation of adaptive immunity.
B cell (B lymphocyte) — An immune cell that, upon activation by encountering its specific antigen, differentiates into plasma cells that produce antibodies. Some B cells become memory B cells — long-lived cells that persist after the immune response and enable rapid antibody production upon re-exposure to the antigen. B cells are the source of the "humoral" (antibody-based) immune response.
T cell (T lymphocyte) — A class of immune cell with multiple roles. Helper T cells (CD4+) coordinate the immune response by activating B cells and cytotoxic T cells. Cytotoxic T cells (CD8+) directly kill infected cells by recognizing foreign antigens on their surface. Memory T cells persist after infection and enable rapid cellular immune responses upon re-exposure.
Innate immunity — The immune system's first, non-specific line of defense. Responds rapidly (within hours) to infection using pattern recognition receptors that detect common features of pathogens. Includes physical barriers (skin, mucus), inflammatory responses, natural killer cells, and complement proteins. The innate response is the initial alarm system; it does not have memory.
Adaptive immunity — The immune system's specific, memory-forming second line of defense. Takes days to weeks to develop initially but provides lasting protection. Requires identifying specific antigens, activating the corresponding B and T cells, and forming memory cells. Vaccines train the adaptive immune system.
Memory cells — Long-lived B and T cells formed during an initial immune response that persist for years or decades. Memory cells can be rapidly activated by re-exposure to their specific antigen, enabling a much faster and stronger immune response than the first exposure triggered — often neutralizing the pathogen before symptoms develop.
Herd immunity — The indirect protection of susceptible individuals that occurs when a sufficient proportion of a population is immune (through vaccination or previous infection) that pathogen transmission is interrupted. When the basic reproduction number (R0) is above 1 but the effective reproduction number is below 1 due to immunity, outbreaks cannot be sustained. The threshold proportion immune required for herd immunity equals 1 − 1/R0.
Adjuvant — A substance added to vaccines to enhance the immune response. Adjuvants activate the innate immune system, increasing inflammation at the injection site and stimulating stronger antigen presentation to adaptive immune cells. Common adjuvants include aluminum salts (alum), AS04, and MF59. The immune response to adjuvanted vaccines is typically stronger and more durable.
mRNA (messenger RNA) — A molecule that carries genetic instructions from DNA to ribosomes, where proteins are synthesized. mRNA vaccines deliver synthetic mRNA encoding a specific viral protein; cells that take up the mRNA produce the protein, which the immune system recognizes as foreign and mounts a response against. The mRNA is degraded within days; it does not enter the cell nucleus or alter DNA.
How the Immune System Responds to a Vaccine
Step 1: Antigen Introduction
A vaccine is administered — typically by injection, though some vaccines are oral or nasal. The vaccine delivers antigens: proteins, protein fragments, weakened or inactivated pathogens, or in the case of mRNA vaccines, instructions to produce a specific protein.
At the injection site, immune cells of the innate system are the first responders. Dendritic cells engulf antigens and migrate to the nearest lymph node. Inflammation at the injection site — the soreness, swelling, and redness after vaccination — is the innate immune response activating. This is expected and indicates the vaccine is working.
The innate immune response also generates danger signals through pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs), which detect molecular patterns common to pathogens. These signals amplify the alarm and are part of why adjuvants — which often mimic pathogen-associated molecular patterns — improve vaccine efficacy. Without sufficient danger signaling, the adaptive immune response may be weak and short-lived.
Step 2: Antigen Presentation
In the lymph node, dendritic cells present antigen fragments on their surface (using MHC — major histocompatibility complex — molecules), displaying them to T cells. The T cell whose receptor matches the specific antigen becomes activated.
This process is exquisitely selective. The human body contains an estimated 25 million to over 1 billion distinct T cell clones, each with a receptor tuned to a different antigen (Davis & Bjorkman, 1988). The chance that any given T cell recognizes a randomly selected antigen is vanishingly small — but the sheer diversity of the T cell repertoire means that for virtually any pathogen, at least some T cells will be a match.
Step 3: B Cell Activation and Antibody Production
Helper T cells (activated in Step 2) activate B cells that carry surface receptors matching the antigen. Activated B cells proliferate and differentiate into plasma cells — antibody factories that produce thousands of antibodies per second, each specific to the vaccine antigen.
These antibodies circulate in the blood and lymphatic system, ready to bind to the antigen if it appears again. Binding of antibody to antigen can directly neutralize pathogens (preventing them from entering cells), mark them for phagocytosis (destruction by macrophages), or activate the complement system.
Somatic hypermutation and affinity maturation are additional refinements: during B cell proliferation in lymph node germinal centers, the genes encoding antibodies undergo rapid mutation, and B cells whose mutant receptors bind antigens more tightly outcompete those that bind less tightly. Over days, the average antibody affinity for the antigen increases — the immune response sharpens itself like a competitive selection process (Abbas, Lichtman & Pillai, 2021).
Step 4: Memory Formation
After the acute immune response subsides, most plasma cells and effector T cells die. But a proportion become memory B cells and memory T cells — long-lived cells that persist for years or decades, circulating in the blood and lymphoid tissues.
These memory cells are the source of vaccine-mediated immunity. Upon re-exposure to the antigen (real infection), memory B cells rapidly differentiate into plasma cells producing large quantities of antibodies within 24-72 hours — far faster than the initial 1-2 week response. Memory T cells can immediately recognize and kill infected cells. This rapid, strong secondary response typically eliminates the pathogen before symptoms develop, or produces only mild illness.
The longevity of immunological memory varies by vaccine and disease. Some vaccines, like the yellow fever vaccine, appear to confer lifelong immunity from a single dose — studies of yellow fever-vaccinated individuals from the 1930s found detectable antibody titers over 75 years later (Gotuzzo et al., 2013). Others, like the seasonal influenza vaccine, produce immunity that wanes within a single year — both because the virus mutates rapidly and because of natural antibody decline.
Booster Doses
The initial vaccine series establishes memory cells. Over time, memory B cells and antibody levels may wane — the immune system maintains less active surveillance of old antigens. Booster doses re-expose the immune system to the antigen, restimulating memory cells and refreshing antibody levels. The response to a booster is faster and stronger than to the initial dose (anamnestic response).
For some vaccines, like the hepatitis B vaccine, a three-dose primary series produces immunity that lasts at least 20-30 years, and a booster is typically not required. For others, like the COVID-19 vaccines, antibody levels against the original strain waned significantly within six months of primary vaccination, and boosters were introduced to maintain protection — particularly against severe disease in elderly and immunocompromised populations (Andrews et al., 2022).
Types of Vaccines
Different vaccine technologies deliver antigens through different mechanisms:
Live-Attenuated Vaccines
Live-attenuated vaccines contain weakened (attenuated) but living pathogen strains. The attenuated strain can replicate in the body and cause a mild, harmless infection, but cannot cause the full disease in immunocompetent individuals.
Attenuation is typically achieved by repeatedly growing the pathogen in non-human cells at lower temperatures, selecting for strains adapted to grow in those conditions and that have lost key virulence factors for the human host. Sabin's oral polio vaccine was attenuated by serial passage in monkey kidney cells. The MMR vaccine viruses were similarly developed.
Advantages: Typically produce robust, long-lasting immunity — sometimes a single dose confers lifetime protection. The immune response closely mimics natural infection.
Disadvantages: Require refrigeration (cold chain); may cause disease in immunocompromised individuals; cannot be given to pregnant women for some formulations.
Examples: MMR (measles, mumps, rubella), varicella (chickenpox), yellow fever, oral polio vaccine (OPV), rotavirus.
Inactivated Vaccines
Inactivated vaccines use pathogens that have been killed by heat, chemicals, or radiation. The killed pathogen cannot replicate but retains its antigenic structure, triggering an immune response.
Advantages: Stable; can be given to immunocompromised individuals; no risk of reversion to virulence.
Disadvantages: Often require multiple doses and adjuvants; typically produce weaker immunity than live vaccines; may require more frequent boosters.
Examples: Influenza (injected), inactivated polio vaccine (IPV), hepatitis A, rabies.
Subunit/Protein Vaccines
Subunit vaccines use specific pieces of a pathogen — typically surface proteins — rather than the whole organism. The immune system responds to the specific protein without any exposure to the complete pathogen.
The hepatitis B vaccine, for example, uses only the hepatitis B surface antigen (HBsAg) — a protein from the virus's outer coat — produced by recombinant yeast cells. Because only one protein is used, the immune response is highly targeted and the vaccine is exceptionally safe.
Advantages: Very safe; no risk of infection; stable.
Disadvantages: May produce weaker immune responses; often require adjuvants and multiple doses.
Examples: Hepatitis B, HPV, pertussis component (in DTaP), meningococcal, pneumococcal.
Toxoid Vaccines
Toxoid vaccines use inactivated bacterial toxins. When the immune response to the disease comes primarily from toxins (rather than the bacteria themselves), immunizing against the toxin prevents disease.
Examples: Tetanus, diphtheria.
mRNA Vaccines
mRNA vaccines deliver synthetic messenger RNA encoding a specific viral protein. Cells at the injection site take up the mRNA, use it to produce the target protein (the SARS-CoV-2 spike protein in COVID-19 vaccines), and display it on their surface. The immune system recognizes the foreign protein and mounts a response.
The mRNA is encapsulated in lipid nanoparticles (LNPs) — tiny fat bubbles that protect the fragile RNA from enzymatic degradation and help it enter cells. This delivery mechanism was a key technical breakthrough that made mRNA vaccines practical; earlier attempts struggled with RNA's instability and the difficulty of cellular uptake.
Advantages: Rapid development time (the mRNA sequence can be designed quickly once the pathogen genome is known); the mRNA does not enter the nucleus or alter DNA; can be updated quickly for new variants; scalable manufacturing.
Disadvantages: Requires cold storage; newer technology with shorter track record (though the underlying mRNA biology has been studied for decades).
Examples: COVID-19 vaccines (Pfizer-BioNTech, Moderna).
"The speed with which mRNA vaccines were developed was not reckless — it was the result of decades of foundational research on RNA biology and lipid nanoparticle delivery that most people had never heard of." — Katalin Kariko, senior vice president at BioNTech, co-recipient of the 2023 Nobel Prize in Physiology or Medicine (paraphrased from Nobel Prize lecture, 2023)
Viral Vector Vaccines
Viral vector vaccines use a modified, harmless virus (often an adenovirus) to deliver genetic instructions for producing a target antigen. The adenovirus enters cells and delivers DNA encoding the antigen; the cell produces the antigen, triggering an immune response.
Examples: AstraZeneca and Johnson & Johnson COVID-19 vaccines, Ebola vaccine (rVSV-ZEBOV).
Protein Subunit and VLP Vaccines (Next Generation)
Virus-like particle (VLP) vaccines consist of protein structures that mimic the shape of a virus without containing any viral genetic material. The HPV vaccines (Gardasil, Cervarix) use VLPs made from the HPV capsid protein. Because VLPs closely resemble the real virus in structure but are non-infectious, they generate strong immune responses including B cell and T cell immunity.
The next generation of vaccine research includes self-amplifying RNA vaccines (which amplify themselves inside cells, enabling lower doses) and nanoparticle antigen display platforms that present multiple antigens in geometrically optimized arrangements to maximize immune response.
Herd Immunity: The Mathematics of Protection
Herd immunity is the phenomenon by which high vaccination rates protect even unvaccinated individuals. The mathematics depend on the basic reproduction number (R0) — the average number of people an infected person infects in a fully susceptible population.
For herd immunity, the effective reproduction number must fall below 1. The threshold vaccination coverage p required is:
p = 1 − 1/R0
| Disease | R0 (approx.) | Herd Immunity Threshold | Vaccine Used |
|---|---|---|---|
| Measles | 12-18 | 92-95% | MMR (live-attenuated) |
| Whooping cough | 12-17 | 92-94% | DTaP (acellular subunit) |
| Mumps | 4-7 | 75-86% | MMR (live-attenuated) |
| Smallpox | 5-7 | 80-86% | Vaccinia (live-attenuated) — eradicated 1980 |
| Polio | 5-7 | 80-86% | OPV or IPV |
| Seasonal flu | 2-3 | 50-67% | Inactivated/recombinant |
| COVID-19 (original) | 2-3 | 50-67% | mRNA / viral vector |
| COVID-19 (Delta) | 5-8 | 80-88% | mRNA / viral vector |
| COVID-19 (Omicron) | 8-15 | 88-93% | mRNA / viral vector |
Measles requires approximately 95% immunity because each infected person, on average, infects 12-18 others in a susceptible population. Only when 95% of the population is immune does each infection produce less than one subsequent infection on average, allowing outbreaks to die out.
The existence of individuals who cannot be vaccinated — immunocompromised patients, newborns, those with specific medical contraindications — means that herd immunity through vaccination protects these vulnerable populations indirectly, as the pathogen cannot reach them through chains of transmission that are interrupted by widespread immunity.
This principle also explains why declining vaccination rates in well-off countries — for ideological rather than medical reasons — have consequences beyond the individuals who decline. When measles vaccination coverage in a community drops from 97% to 90%, it falls below the herd immunity threshold. Outbreaks become possible not just among the unvaccinated but among the medically vulnerable who depend on community immunity for protection. The 2019 US measles outbreak, which reached over 1,200 cases — the highest in 27 years — was concentrated in communities with low vaccination rates (CDC, 2019).
Vaccine Safety and Development
Vaccine development follows a rigorously staged process before approval:
Preclinical research — Laboratory and animal testing to identify candidate vaccine antigens, test for basic safety, and establish proof of concept. Most candidates fail at this stage.
Phase I trials — Safety testing in small groups (20-100 people). Primary goal is to identify safety signals and optimal dose. Efficacy is not assessed.
Phase II trials — Safety and preliminary efficacy in hundreds of people. Begins to identify whether the vaccine generates an immune response and at what dose.
Phase III trials — Definitive efficacy and safety in thousands to tens of thousands of people. Compares vaccinated versus unvaccinated groups (typically randomized, double-blind, placebo-controlled) to measure protection against disease. Also powered to detect uncommon adverse events.
Regulatory review — Agencies such as the FDA, EMA, and WHO review all trial data before approving or authorizing a vaccine for use.
Phase IV surveillance — Ongoing safety monitoring post-approval via pharmacovigilance systems (VAERS in the US, Yellow Card in the UK) and active surveillance studies. This stage detects very rare adverse events (those occurring in fewer than 1 in 50,000 or 1 in 1 million vaccinees) that cannot be detected in even large Phase III trials.
This process typically takes 10-15 years for conventional vaccines. COVID-19 vaccines were developed in under a year because: the SARS-CoV-2 genome was published in January 2020 enabling rapid antigen design; mRNA technology had been in development for decades; regulatory agencies allowed parallel phase transitions rather than sequential; and governments pre-ordered billions of doses before approval, removing financial risk and allowing manufacturing scale-up during trials. No trial phase was skipped — they were compressed and overlapped.
The Signal-to-Noise Challenge in Safety Monitoring
Side effects after vaccination — soreness, fatigue, fever — are typically the immune system responding to the antigen, not the disease itself. These reactogenic effects are generally benign and transient.
Serious adverse effects are rare. For the mRNA COVID-19 vaccines, myocarditis (inflammation of the heart muscle) was identified in post-approval surveillance, occurring primarily in young males after the second dose at a rate of approximately 1-4.8 cases per 100,000 doses in the highest-risk group (16-29 year old males). Cases were generally mild and resolved quickly. The background rate of myocarditis in the same demographic from COVID-19 infection itself was substantially higher — estimated at 11-16 per 100,000 among those infected — meaning the vaccine risk was far lower than the disease risk (Bozkurt et al., 2021).
For the adenoviral vector COVID-19 vaccines, vaccine-induced immune thrombocytopenia and thrombosis (VITT) — a rare clotting disorder — was identified at a rate of approximately 1 per 100,000 doses. This led several countries to restrict AstraZeneca use in younger age groups where the risk-benefit calculation was less favorable than in older populations at higher COVID-19 risk.
These safety signals were detected, investigated, and acted upon within months of deployment — a demonstration of the pharmacovigilance system functioning as designed.
Vaccine Efficacy vs. Effectiveness
A critical distinction often lost in public communication is the difference between vaccine efficacy and vaccine effectiveness.
Vaccine efficacy is measured in randomized controlled trials under controlled conditions. It answers: among people randomly assigned to receive the vaccine versus placebo, what percentage reduction in disease incidence occurred in the vaccine group?
Vaccine effectiveness is measured in observational studies in real-world conditions. It answers: among people who were vaccinated versus unvaccinated, what was the reduction in disease outcomes in actual practice?
Real-world effectiveness often differs from clinical trial efficacy because: the population receiving the vaccine differs from trial participants; conditions of storage and administration vary; immune responses vary across populations; and the circulating pathogen strain may differ from the strain used in trials.
The Pfizer-BioNTech COVID-19 vaccine had reported efficacy of 95% against the original Wuhan strain in Phase III trials (Polack et al., 2020). Real-world effectiveness against severe disease and hospitalization remained high even as effectiveness against infection declined substantially with Omicron — reflecting both immune waning and antigenic mismatch between the original vaccine and the new variant.
Global Vaccine Access and Equity
The development of safe, effective vaccines is only the first challenge. Distributing them equitably across a world of vastly unequal resources is a separate and deeply political problem.
The COVID-19 pandemic exposed stark inequities in vaccine access. By the end of 2021, high-income countries had administered enough doses to fully vaccinate their populations multiple times over, while many low-income countries had vaccinated less than 5% of their populations. A study in Nature Medicine estimated that between December 2020 and December 2021, COVID-19 vaccines prevented 19.8 million deaths globally — but noted that better distribution of vaccines to high-mortality, under-vaccinated settings could have prevented an additional 600,000 deaths (Watson et al., 2022).
The COVAX initiative — a WHO-led mechanism to pool vaccine procurement and distribute doses to lower-income countries — was intended to prevent vaccine nationalism but was overwhelmed by wealthy-country bilateral deals that secured supply ahead of the global mechanism.
Cold chain logistics remain a significant constraint for vaccine delivery in low-resource settings. Vaccines that require storage at -70°C (like early mRNA formulations) are impractical without refrigerated supply chains; vaccines requiring only 2-8°C refrigeration or room-temperature stability are more deployable in resource-limited environments. This has driven investment in thermostable vaccine formulations and novel delivery systems including microneedle patches that can be administered without syringes.
The History of Vaccination in Brief
The 225-year history of vaccination is a story of incremental understanding gradually catching up to empirical practice.
1796: Jenner's cowpox-to-smallpox protection demonstration. No understanding of mechanism.
1880s: Louis Pasteur develops vaccines for chicken cholera, anthrax, and rabies, establishing the principle of using attenuated organisms. Coins the term "vaccine" in honor of Jenner (from vacca, Latin for cow).
1890s: Emil von Behring and Shibasaburo Kitasato demonstrate passive immunity via transfer of serum from immunized animals — the foundation of antibody biology. Von Behring receives the first Nobel Prize in Physiology or Medicine (1901).
1930s-1950s: Development of influenza, yellow fever, and early polio vaccines. Jonas Salk's inactivated polio vaccine (1955) and Albert Sabin's oral live-attenuated vaccine (1961) trigger mass vaccination campaigns that reduce global polio cases by over 99%.
1980: Smallpox declared eradicated — the first and only human disease ever eliminated from the planet by vaccination.
1990s: Hepatitis B and recombinant subunit vaccines; development of meningococcal conjugate vaccines.
2006: HPV vaccines approved — the first cancer-prevention vaccines, targeting the human papillomavirus that causes approximately 95% of cervical cancers.
2020-2021: mRNA vaccine technology achieves its first widespread deployment with COVID-19 vaccines, compressing the vaccine development timeline from years to months.
References
- Jenner, E. (1798). An Inquiry into the Causes and Effects of the Variolae Vaccinae. Low.
- Abbas, A. K., Lichtman, A. H., & Pillai, S. (2021). Cellular and Molecular Immunology (10th ed.). Elsevier.
- Plotkin, S. A., Orenstein, W. A., Offit, P. A., & Edwards, K. M. (Eds.) (2018). Plotkin's Vaccines (7th ed.). Elsevier.
- Davis, M. M., & Bjorkman, P. J. (1988). T-cell antigen receptor genes and T-cell recognition. Nature, 334(6181), 395-402. https://doi.org/10.1038/334395a0
- Pollard, A. J., & Bijker, E. M. (2021). A guide to vaccinology: from basic principles to new developments. Nature Reviews Immunology, 21, 83-100. https://doi.org/10.1038/s41577-020-00479-7
- Polack, F. P., et al. (2020). Safety and efficacy of the BNT162b2 mRNA COVID-19 vaccine. New England Journal of Medicine, 383(27), 2603-2615. https://doi.org/10.1056/NEJMoa2034577
- Andrews, N., et al. (2022). COVID-19 vaccine effectiveness against the Omicron (B.1.1.529) variant. New England Journal of Medicine, 386, 1532-1546. https://doi.org/10.1056/NEJMoa2119451
- Bozkurt, B., et al. (2021). Myocarditis with COVID-19 mRNA vaccines. Circulation, 144(6), 471-484. https://doi.org/10.1161/CIRCULATIONAHA.121.056135
- Watson, O. J., et al. (2022). Global impact of the first year of COVID-19 vaccination: a mathematical modelling study. The Lancet Infectious Diseases, 22(9), 1293-1302. https://doi.org/10.1016/S1473-3099(22)00320-6
- Gotuzzo, E., et al. (2013). Efficacy and duration of immunity after yellow fever vaccination: systematic review on the need for a booster every 10 years. American Journal of Tropical Medicine and Hygiene, 89(3), 434-444. https://doi.org/10.4269/ajtmh.13-0264
- Santoli, J. M., et al. (2020). Effects of the COVID-19 pandemic on routine pediatric vaccine ordering and administration. MMWR Morbidity and Mortality Weekly Report, 69(19), 591-593. https://doi.org/10.15585/mmwr.mm6919e2
- Centers for Disease Control and Prevention. (2019). Measles Cases and Outbreaks. CDC. https://www.cdc.gov/measles/cases-outbreaks.html
- WHO. (2022). Vaccines and Immunization. World Health Organization. https://www.who.int/health-topics/vaccines-and-immunization
- Hollingsworth, T. D., et al. (2011). Mitigation strategies for pandemic influenza A: balancing conflicting policy objectives. PLOS Computational Biology, 7(2). https://doi.org/10.1371/journal.pcbi.1001076
For related concepts, see how the immune system works, how pandemics spread, and herd immunity explained.
Frequently Asked Questions
How do vaccines work?
Vaccines work by introducing a harmless form of a pathogen — or a piece of it — to the immune system, triggering an immune response and the formation of memory cells. When the real pathogen later appears, the immune system recognizes it immediately and mounts a rapid, strong response, neutralizing the threat before it causes disease. Vaccines train the immune system without the risks of actual infection.
What is the difference between B cells and T cells in immunity?
B cells (B lymphocytes) produce antibodies — proteins that bind specifically to antigens on pathogens, neutralizing them and marking them for destruction. T cells (T lymphocytes) have two main types: helper T cells coordinate the immune response, and cytotoxic (killer) T cells directly destroy infected cells. Memory B cells and memory T cells persist after the initial response and enable rapid immunity upon re-exposure.
How do mRNA vaccines work?
mRNA vaccines (like the COVID-19 vaccines from Pfizer-BioNTech and Moderna) deliver messenger RNA instructions that tell your cells to produce a specific viral protein — for COVID-19, the spike protein. Your immune system recognizes this protein as foreign, mounts a response, and forms memory cells. The mRNA is degraded within days and never enters the cell nucleus or interacts with DNA. You never receive the actual virus.
What is herd immunity?
Herd immunity occurs when enough of a population is immune (through vaccination or previous infection) that pathogens cannot spread effectively — even people who are not immune are protected because the pathogen rarely reaches them. The threshold varies by how contagious the disease is: measles (highly contagious) requires about 95% immunity; polio requires about 80-85%. Below the threshold, outbreaks can occur even in populations with significant immunity.
What are the different types of vaccines?
Live-attenuated vaccines use weakened but living pathogens (MMR, chickenpox, yellow fever). Inactivated vaccines use killed pathogens (flu shot, polio IPV, hepatitis A). Subunit/protein vaccines use specific pieces of the pathogen (hepatitis B, HPV, pertussis component). Toxoid vaccines use inactivated toxins (tetanus, diphtheria). Viral vector vaccines use a modified virus to deliver genetic instructions (some COVID-19 vaccines, Ebola). mRNA vaccines deliver direct genetic instructions (COVID-19 Pfizer, Moderna). Each approach has different advantages in terms of immune response strength, stability, and manufacturing.
Why do some vaccines require multiple doses?
Multiple doses serve different purposes. A primary series of doses (e.g., 3 doses for hepatitis B) builds the initial immune response progressively — each dose boosts the response further. Boosters after months or years address the natural waning of antibody levels over time, topping up immunity. Some vaccines require two doses to achieve full protection because the immune response from one dose alone is insufficient for durable immunity.