Vaccines Overview

Last reviewed: February 22, 2022

On this page:

• Table
• Overview
• Efficacy
• Resources
• Multimedia

COVID-19 Vaccines with Peer-Reviewed Published U.S. Clinical Efficacy Data

Overview

Vaccination to prevent disease was first conceptualized in the late 18th century, and by the early 20th century vaccines for diseases including tuberculosis, yellow fever and influenza had been developed (Plotkin, 2014). By 1980, vaccination had been used to eradicate smallpox globally — one of only two infectious diseases to date to be eliminated from the environment. Numerous vaccines are responsible for preventing millions of illnesses annually (Pardi, 2018).

Conventional vaccine types include the following:

• Live-attenuated vaccines, such as the measles-mumps-rubella vaccine, contain attenuated (weakened) forms of an organism that causes a disease. This attenuated organism acts as an antigen and stimulates the body to create a robust antibody response.
• Inactivated vaccines, including most influenza vaccines, contain a killed version of an organism that causes a disease. This killed form acts as an antigen and stimulates the body to create an antibody response.
• Subunit, recombinant, polysaccharide and conjugate vaccines, such as pneumococcal vaccines, contain components of an organism which act as antigens and stimulate an antibody response. They do not contain the organism itself.
• Toxoid vaccines, such as tetanus vaccine, contain a toxin made by an organism that causes a disease. The toxin acts as an antigen and stimulates an antibody response to specific parts of the organism, rather than the whole organism.

While conventional vaccines are critical in controlling disease, limitations include the time and materials required for production, difficulty with large-scale deployment and a reliance on the adaptive instead of innate immune response (which some infections may evade) (Pardi, 2018).

Newer vaccine technologies that have been deployed for the COVID-19 pandemic include the following:

• Messenger RNA (mRNA) vaccines, such as the Pfizer-BioNTech and Moderna COVID-19 vaccines, which contain synthetic mRNA molecules encoding vaccine antigens encapsulated within nanoparticles. These particles deliver the mRNA directly into the cytoplasm, where it can be transcribed by host ribosomes into vaccine antigens that then elicit an immune response.
• Viral vector vaccines, such as the Johnson & Johnson/Janssen and Oxford-AstraZeneca COVID-19 vaccines (as well as the tetravalent dengue vaccine or recombinant vesicular stomatitis virus–Zaire Ebola virus vaccine), which utilize either non-pathogenic organisms or plasmids (the vector) containing genes coding for proteins that act as vaccine antigens. The vaccine delivers the vector, which infects host cells and then travels to the nucleus, where the genes encoded by the vector are expressed, resulting in the creation of the antigen that induces a host immune response.
• Nanoparticle vaccines, such as the Novavax COVID-19 vaccine, which use nanometer-sized particles to deliver a vaccine antigen. Multiple copies of the antigen may be attached to the nanoparticle to enhance the immune response.

Efficacy

Q: What does “vaccine efficacy” mean, and how was efficacy measured in the Pfizer-BioNTech, Moderna, Johnson & Johnson/Janssen and Oxford-AstraZeneca COVID-19 vaccine trials? 

A: Vaccine efficacy refers to the percent reduction in cases of a disease among individuals who receive a vaccine compared with those who are unvaccinated. The primary efficacy endpoint in all the trials was clinical disease, meaning symptomatic COVID-19; reduction in infection, which would include both symptomatic COVID-19 as well as any positive test for SARS-CoV-2 in the absence of symptoms, was not assessed as a primary endpoint, although additional data utilizing serologic endpoints are being collected in all the trials. When the term “vaccine efficacy” is discussed in relation to these vaccines, it generally refers to efficacy at preventing clinical disease unless otherwise specified. 

The primary endpoints for the Phase 3 trials of these vaccines were as follows: 

• Pfizer-BioNTech: Efficacy against PCR-confirmed symptomatic COVID-19 with onset at least 7 days after the second dose of vaccine among participants without serologic or virologic evidence of prior SARS-CoV-2 infection at baseline. 
• Moderna: Efficacy against PCR-confirmed symptomatic COVID-19 with onset at least 14 days after the second dose of vaccine among participants without evidence of prior SARS-CoV-2 infection at baseline. 
• Johnson & Johnson/Janssen: Efficacy against PCR-confirmed moderate to severe/critical COVID-19 in the periods starting 14 days after vaccination and 28 days after vaccination among participants without evidence of prior SARS-CoV-2 infection at baseline. 
• Oxford-AstraZeneca: Efficacy against PCR-confirmed symptomatic COVID-19 starting 14 days after dose 2 of the vaccine. 

Q: When does immunity to symptomatic SARS-CoV-2 infection develop after completion of a COVID-19 vaccine series? 

A: Our current knowledge regarding when vaccinated persons can expect to achieve a high level of protection from developing symptomatic COVID-19 is derived from the published clinical trial data. The Moderna COVID-19 vaccine demonstrated 95% efficacy for prevention of symptomatic COVID-19 starting 14 days after receiving the second dose; the Pfizer-BioNTech COVID-19 vaccine demonstrated 95% efficacy for prevention of symptomatic COVID-19 starting 7 days after receiving the second dose; the Johnson & Johnson/Janssen COVID-19 vaccine demonstrated 67% efficacy for prevention of moderate-severe/critical COVID-19 starting 14 days after vaccination; finally, the Oxford-AstraZeneca COVID-19 vaccine demonstrated 67% efficacy for prevention of symptomatic COVID-19 starting 14 days after receiving the second dose. 

Resources