Vaccination is the most effective medical intervention for preventing viral diseases, estimated to prevent 2–3 million deaths annually. Viral vaccines work by presenting antigens that mimic a pathogen to the immune system, inducing immunological memory that enables rapid protection upon natural exposure. The diversity of vaccine platforms reflects the different challenges posed by different viruses, including safety, efficacy, durability of protection, and logistical considerations for global distribution.
Live-Attenuated Vaccines
Live-attenuated vaccines contain living viruses that have been weakened through serial passage in cell culture or by genetic engineering so they replicate poorly or not at all in humans while retaining immunogenicity. These vaccines typically induce strong, durable immunity after a single or two doses because they mimic natural infection, stimulating both humoral and cellular immune responses including CD8+ T cell responses. Successful live-attenuated vaccines include the measles, mumps, rubella (MMR) vaccine, the oral polio vaccine (Sabin), the yellow fever vaccine (17D strain), the varicella (chickenpox) vaccine, and the intranasal influenza vaccine (FluMist). The oral polio vaccine has been instrumental in the near-eradication of polio, with wild poliovirus type 2 declared eradicated in 2015 and type 3 in 2019. The primary concern with live-attenuated vaccines is the risk of reversion to virulence, which occurs at very low frequency with the oral polio vaccine, causing vaccine-associated paralytic polio at a rate of approximately 1 in 2.4 million doses. Live-attenuated vaccines are generally contraindicated in immunocompromised individuals.
Inactivated Vaccines
Inactivated vaccines contain virus that has been killed by chemical treatment (formaldehyde or β-propiolactone) or heat, rendering it non-infectious while preserving antigenic structure. Inactivated vaccines are safer than live vaccines because they cannot replicate or revert to virulence, making them suitable for immunocompromised individuals. However, they typically require multiple doses and adjuvants to induce protective immunity, and they predominantly elicit humoral (antibody) responses with weaker T cell responses compared to live vaccines. Examples include the inactivated polio vaccine (Salk), inactivated influenza vaccines, the rabies vaccine, the hepatitis A vaccine, and most whole-virion COVID-19 vaccines (CoronaVac, Sinopharm). Booster doses are often needed to maintain protective antibody levels. Whole-virion inactivated vaccines contain the complete set of viral structural proteins, while some inactivated preparations may be split or subunit vaccines that include only selected antigenic components.
Subunit and Virus-Like Particle Vaccines
Subunit vaccines contain purified viral antigens rather than whole virus, eliminating the risk of infection and reactogenicity associated with whole-virus vaccines. The hepatitis B vaccine, first licensed in 1986, was the first subunit vaccine produced by recombinant DNA technology, consisting of the hepatitis B surface antigen (HBsAg) produced in yeast cells. Virus-like particle (VLP) vaccines are a special class of subunit vaccines in which viral structural proteins self-assemble into particles that mimic the size and structure of native virions but lack genetic material, rendering them non-infectious. The human papillomavirus (HPV) vaccines (Gardasil, Cervarix) are VLP vaccines composed of recombinant L1 capsid protein assembled into VLPs, which are highly immunogenic and have demonstrated remarkable efficacy in preventing HPV infection and cervical cancer. VLP technology has also been applied to hepatitis E, Norovirus, and influenza vaccine development. Subunit vaccines typically require adjuvants to enhance immunogenicity. The hepatitis B vaccine uses aluminum salts (alum), while newer adjuvants such as AS04 (alum with monophosphoryl lipid A) and MF59 (oil-in-water emulsion) are used in HPV and influenza vaccines, respectively.
Viral Vector Vaccines
Viral vector vaccines use a harmless virus (the vector) to deliver genetic material encoding the antigen of interest into host cells, where the antigen is produced endogenously and presented to the immune system. Adenovirus-based vectors are among the most widely used, including the chimpanzee adenovirus ChAdOx1 used in the Oxford-AstraZeneca COVID-19 vaccine and the human adenovirus type 26 (Ad26) used in the Johnson & Johnson COVID-19 vaccine. Vesicular stomatitis virus (VSV) vectors are used in the Ebola vaccine (rVSV-ZEBOV), which demonstrated 100% efficacy in clinical trials. Viral vectors can induce strong T cell and antibody responses, and pre-existing immunity to the vector (particularly human adenoviruses) can reduce vaccine immunogenicity, a limitation addressed by using rare serotypes or non-human primate adenoviruses. Modified vaccinia Ankara (MVA) and canarypox vectors (ALVAC) are additional poxvirus-based platforms with excellent safety profiles that are used in vaccines against HIV, malaria, and tuberculosis under development.
Nucleic Acid Vaccines
Nucleic acid vaccines deliver genetic material (DNA or RNA) encoding the antigen directly to host cells, leveraging the cellular machinery to produce the antigen in situ. Messenger RNA (mRNA) vaccines, validated at unprecedented speed during the COVID-19 pandemic, consist of synthetic mRNA encoding the viral antigen encapsulated in lipid nanoparticles (LNPs) for delivery and protection against nucleases. The Pfizer-BioNTech (BNT162b2) and Moderna (mRNA-1273) COVID-19 vaccines achieved over 90% efficacy in clinical trials and have been administered to billions of people worldwide. mRNA vaccines offer several advantages: they can be designed and manufactured rapidly based on genomic sequence information alone, they are produced entirely in cell-free systems, and they do not integrate into the host genome. Modified nucleosides (such as N1-methylpseudouridine) reduce innate immune sensing and enhance translation. The LNP formulation is critical for delivery, stability, and immunogenicity. DNA vaccines consist of plasmid DNA encoding the antigen, delivered by electroporation or needle-free injection, and offer greater stability than mRNA but generally lower immunogenicity in humans.
Vaccination Strategies and Herd Immunity
Vaccination strategies aim to achieve herd immunity, the threshold at which a sufficient proportion of the population is immune that transmission is interrupted, protecting even unvaccinated individuals through reduced pathogen circulation. The herd immunity threshold depends on the basic reproduction number (R₀) of the pathogen and vaccine effectiveness, and for measles (R₀ = 12–18) requires approximately 95% of the population to be immune. Routine childhood immunization programs are the foundation of public health vaccination, achieving high coverage for diseases including measles, polio, diphtheria, tetanus, pertussis, and hepatitis B. Booster doses are required when immunity wanes over time or when antigenic variation allows pathogen escape, as with seasonal influenza vaccines updated annually to match circulating strains. Maternal immunization, where pregnant women are vaccinated to protect both themselves and transfer protective antibodies to the newborn, is effective against influenza, pertussis, and more recently RSV. Ring vaccination, targeted vaccination of contacts of confirmed cases, was used successfully in smallpox eradication and the Ebola outbreak response.
