SARS-CoV-2 vaccines are medical products developed to protect individuals from coronavirus disease 2019 (COVID-19), an illness caused by the SARS-CoV-2 virus. Their primary purpose is to prepare the human immune system to recognize and fight the virus without causing the actual illness.
What SARS-CoV-2 Vaccines Are
SARS-CoV-2 vaccines introduce a harmless component of the virus to the body, allowing the immune system to learn how to defend against future infections. This component is typically the spike (S) protein, found on the virus’s surface and responsible for its entry into human cells. The vaccines do not contain the live virus, so they cannot cause COVID-19. Instead, they provide the blueprint or pieces of the spike protein, training the immune system to identify the threat and form immunological memory. This memory allows the body to respond quickly if it encounters the actual virus later.
How SARS-CoV-2 Vaccines Work
Different scientific approaches are used in SARS-CoV-2 vaccine development, each presenting the spike protein to the immune system distinctly. The most common types include messenger RNA (mRNA) vaccines, viral vector vaccines, and protein subunit vaccines. Each type stimulates the immune system to produce antibodies and T-cells, specialized immune cells that target infected cells.
mRNA Vaccines
mRNA vaccines, such as those from Pfizer-BioNTech and Moderna, contain genetic instructions (messenger RNA) for making the SARS-CoV-2 spike protein. When injected, muscle cells absorb this mRNA and use their machinery to produce copies of the spike protein. The immune system then recognizes these proteins as foreign and produces antibodies and activates T-cells against them. The mRNA is temporary and breaks down after delivering its instructions, without entering the cell’s nucleus or altering DNA.
Viral Vector Vaccines
Viral vector vaccines, like the Janssen (Johnson & Johnson) vaccine, use a modified, harmless virus (often an adenovirus) as a delivery system. This vector carries genetic material that instructs human cells to produce the SARS-CoV-2 spike protein. Once the spike protein is made and displayed on cell surfaces, the immune system responds by generating antibodies and defensive white blood cells.
Protein Subunit Vaccines
Protein subunit vaccines, exemplified by Novavax, directly introduce purified pieces of the SARS-CoV-2 spike protein into the body. These vaccines may also include an adjuvant, an ingredient that enhances the immune response. Upon vaccination, the immune system recognizes these proteins and produces antibodies and activates T-cells.
Safety and Monitoring
The development of SARS-CoV-2 vaccines followed a rigorous scientific process, including multiple phases of clinical trials to ensure their safety and effectiveness. This process begins with preclinical assessment in laboratories and animal models. If promising, a vaccine candidate progresses to human testing.
Clinical Trial Phases
Clinical trials are conducted in three phases:
Phase 1 trials involve a small group of healthy volunteers (dozens) to assess initial safety, dosage, and immune response.
Phase 2 expands to hundreds of participants, further evaluating safety and immune responses across a diverse group, and determining the most effective dose.
Phase 3 trials are large-scale studies (thousands of volunteers) to confirm safety, assess effectiveness against the disease, and identify less common side effects.
Regulatory Approval
Following successful clinical trials, regulatory bodies like the U.S. Food and Drug Administration (FDA) review all collected data before authorizing or approving a vaccine for public use. This includes detailed reviews of manufacturing processes and quality control. Initially, some vaccines received Emergency Use Authorizations (EUAs) during the pandemic, allowing rapid deployment while meeting high safety and effectiveness standards. Full approval, like that granted to Comirnaty (Pfizer-BioNTech), requires even more extensive data and follow-up.
Post-Market Surveillance
Post-market surveillance systems continuously monitor vaccine safety once they are in widespread use. The Vaccine Adverse Event Reporting System (VAERS), co-managed by the CDC and FDA, collects reports of potential side effects. Another system, v-safe, is a smartphone-based tool for vaccine recipients to report their health and symptoms. These systems help identify unexpected or rare adverse events for further investigation, such as myocarditis or pericarditis (rare heart inflammations observed more often in young males after mRNA vaccines) or very rare severe allergic reactions (anaphylaxis). Common, typically mild and temporary side effects include pain, redness, or swelling at the injection site, fever, fatigue, headache, muscle pain, and chills, which usually resolve within a few days.
The Public Health Impact of Vaccination
SARS-CoV-2 vaccination has significantly reduced the burden of COVID-19. Studies consistently show that vaccinated individuals have a lower risk of severe illness, hospitalizations, and death compared to unvaccinated individuals. This protection extends across various age groups, including older adults at higher risk of severe outcomes. For example, a 10% increase in vaccination coverage has been associated with an 8% reduction in mortality rates and a 7% reduction in case incidence at the population level.
Vaccination also reduces virus transmission within communities. By lowering the likelihood of infection and the amount of virus an individual carries, vaccines help interrupt chains of transmission. This contributes to community protection, sometimes called population immunity, which helps safeguard vulnerable individuals unable to receive vaccines due to health conditions. While achieving classic “herd immunity” against SARS-CoV-2 is complex due to evolving variants and waning immunity, widespread vaccination still limits viral spread.
The broader societal benefits of vaccination are extensive. Reduced rates of severe illness and hospitalization lessen the strain on healthcare systems, preserving resources and capacity for other medical needs. Widespread vaccination also facilitates a return to more normal social and economic activities, contributing to economic recovery and improved overall societal well-being.