Immunology
Vaccination
Live, killed, subunit, conjugate, toxoid, mRNA — training adaptive immunity to recognize and remember pathogens
Vaccines deliver antigens that prime the adaptive immune system to recognize a pathogen without causing disease, generating memory B and T cells that respond rapidly to future exposure. Live attenuated (MMR, varicella, yellow fever, oral polio, BCG): replicate weakly, induce strong durable immunity, contraindicated in immunocompromise and pregnancy. Inactivated/killed (influenza, hepatitis A, rabies, Salk polio): cannot replicate, often need boosters. Subunit/recombinant (hepatitis B, HPV, acellular pertussis): purified antigen. Conjugate (Hib, pneumococcal, meningococcal): polysaccharide linked to carrier protein for T-cell help in infants. Toxoid (tetanus, diphtheria): inactivated toxin. mRNA (COVID-19): lipid nanoparticles deliver mRNA encoding antigen. Vaccines have eradicated smallpox, eliminated polio in most countries, and prevent ~3.5 million deaths yearly.
- Smallpox eradicationDeclared 1980 (Jenner started 1796)
- COVID-19 mRNA vaccinesFirst authorized December 2020
- Herd immunity threshold (measles)~95% (R0 ~12-18)
- Pediatric schedule (US)~14 vaccines, 50+ doses by age 18
- Annual deaths prevented~3.5 million globally (WHO)
- First vaccineEdward Jenner cowpox → smallpox (1796)
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Why vaccination matters
- Disease eradication and elimination. Smallpox eradicated; polio at the cusp; measles eliminated regionally before resurgence.
- Herd immunity protection. Vaccinated majority shields infants, transplant recipients, and pregnant women.
- Cancer prevention. HPV prevents cervical/anogenital cancers; hepatitis B prevents hepatocellular carcinoma.
- Pandemic response. mRNA platforms produced authorized COVID vaccines in <12 months — saved millions of lives.
- Antibiotic stewardship. Pneumococcal and Hib vaccines reduce serious bacterial infections and antibiotic exposure.
- Travel medicine. Yellow fever, typhoid, Japanese encephalitis vaccines protect international travelers.
- Cost-effectiveness. Childhood vaccines return ~$10-50 in healthcare savings per dollar invested.
Common misconceptions
- Vaccines cause autism. Multiple large studies refuted this; the original Wakefield paper was retracted for fraud.
- Natural infection is better than vaccination. Natural infection often gives durable immunity but at the cost of severe disease and death the vaccine prevents.
- Vaccines overload the infant immune system. Infants respond to thousands of antigens daily; a full schedule uses a tiny fraction.
- If everyone else is vaccinated, my child is safe unvaccinated. Falling coverage erodes herd immunity, with measles outbreaks already returning.
- mRNA vaccines alter DNA. mRNA doesn't enter the nucleus and lacks reverse transcriptase; no integration.
- Annual flu shot causes flu. Inactivated influenza vaccine cannot cause infection; symptoms after vaccination are short-lived immune activation.
Frequently asked questions
How do mRNA vaccines work?
Lipid nanoparticles deliver mRNA encoding the pathogen's antigen (SARS-CoV-2 spike protein for COVID-19) to muscle cells. Cells translate the mRNA into protein, display fragments on MHC I and present whole protein to B cells. Adaptive immunity generates anti-spike antibodies and CD8 T cells. mRNA degrades within days; no genomic integration occurs (mRNA cannot enter nucleus or reverse-transcribe without retroviral machinery). Advantages: rapid design (days from sequence to candidate), strong T cell responses, no live virus. Pfizer-BioNTech and Moderna mRNA COVID vaccines were >94% effective against severe disease initially.
What is herd immunity?
When enough of a population is immune that an introduced infection cannot sustain transmission, protecting unvaccinated individuals indirectly. Threshold = 1 - 1/R0. Measles (R0 ~15): ~95% needed. Polio (R0 ~6): ~80%. COVID-19 with original strain (R0 ~3): ~70%; with Omicron (R0 ~10) and waning immunity, traditional herd immunity proved unattainable. Herd immunity protects infants, immunocompromised, and pregnant women who cannot be vaccinated. Falling MMR coverage drives recurring measles outbreaks in undervaccinated communities.
Why do some vaccines need boosters?
Memory B and T cell populations decline over time and pathogens evolve. Tetanus toxoid: every 10 years because antitoxin titers wane. Influenza: annual because antigenic drift creates new strains. COVID-19: boosters address waning neutralizing antibodies and variant emergence. Live vaccines (MMR) often confer lifelong immunity. Mucosal pathogens (rotavirus) typically need boosters. Hepatitis B usually does not need boosters in immunocompetent adults — protection lasts decades after seroconversion.
What are vaccine adverse events?
Common: injection-site pain, low-grade fever, fatigue, headache — reflect immune activation. Serious but rare: anaphylaxis (~1 per million doses, treated with epinephrine), Guillain-Barré (~1 per million for some flu vaccines), myocarditis (mRNA COVID vaccines, ~1 per 10,000 in young men, usually mild), febrile seizures (MMR ~1 per 3,000), thrombosis with thrombocytopenia syndrome (adenovirus-vectored COVID vaccines, ~1 per 100,000-million). Benefits massively outweigh risks for almost all populations. VAERS captures reports; CISA evaluates causality.
How are vaccines developed and approved?
Preclinical: cell and animal studies. Phase 1: ~20-100 healthy volunteers, safety and immunogenicity. Phase 2: hundreds, dose-finding. Phase 3: tens of thousands, randomized efficacy trial. FDA review and approval. Phase 4: post-marketing surveillance. Traditional timelines: 10-15 years. COVID-19 mRNA vaccines compressed development through parallel manufacturing, prior coronavirus research, and emergency funding — full clinical trial rigor preserved.
Why are some live vaccines contraindicated?
Live attenuated vaccines (MMR, varicella, yellow fever, BCG, oral polio, rotavirus) replicate weakly. In severely immunocompromised hosts (HIV with low CD4, hematologic malignancy, post-transplant, high-dose steroids, chemotherapy), even attenuated viruses can disseminate and cause disease. Pregnant women avoid live vaccines because of theoretical fetal risk. Inactivated vaccines (flu shot, killed polio, hepatitis B) are safe in these populations and often essential.
What is the HPV vaccine and why is it given to adolescents?
HPV vaccine (Gardasil 9) protects against 9 high-risk and low-risk strains causing cervical, oropharyngeal, anal, vulvar, vaginal, and penile cancer plus genital warts. Recommended at age 9-12 — before sexual debut for maximum benefit and because pre-adolescents mount stronger antibody responses. Two doses if started <15, three if older or immunocompromised. Real-world data show >90% reduction in cervical cancer in vaccinated cohorts. Australia and Sweden are projected to nearly eliminate cervical cancer.