Frequently Asked Questions about Development, Safety, and Effectiveness

Yes, COVID-19 vaccines have a very strong safety record. The COVID-19 vaccines have been tested and monitored for safety more than any previous vaccine in U.S. history. All COVID-19 vaccines have been rigorously tested and reviewed for human safety through a three-phase clinical trial process. 

More than 150,000 people participated in U.S. vaccine clinical trials, and almost 700 million vaccine doses have been safely administered in the U.S., with rare instances of adverse reactions. To ensure the continued safety of COVID-19 vaccines, data from clinical trials is collected and monitored for two years after each vaccine is first administered. Additional monitoring is in place for adverse event reporting through the Vaccine Adverse Event Reporting System (VAERS). Based on these thorough processes, public health officials make evidence-based recommendations to keep the public safe and healthy.

Updated December 14, 2023 

Yes, the COVID-19 vaccines are highly effective at reducing the risk of severe infection, symptomatic illness, hospitalization, and death. On average, the original Moderna, Pfizer-BioNTech, and Novavax COVID-19 vaccines were effective in preventing symptomatic COVID-19 illness by more than 85%. 

A vaccine can only protect against a strain that it recognizes. As the virus continues to circulate and mutate over time, new variants are less recognizable to our immune systems. This leads to new infections — even for people who are vaccinated. As a result, updated vaccines have been developed to more closely target the currently circulating variants of concern and offer increased protection. 

The updated Moderna, Pfizer-BioNTech, and Novavax vaccines that were approved in fall 2023 are critical to collective immunity. According to the latest research data, the newly updated COVID-19 vaccines are effective at producing strong immune responses to new variants, especially those originating from the Omicron variant. Individuals who received the updated COVID-19 vaccines have shown antibody responses that were almost 10 to 17 times higher against new variants than before they received the updated vaccine.

Added December 14, 2023 

There are two key measures that determine vaccine effectiveness: clinical trial efficacy and real-world effectiveness

  • Trial efficacy is measured in a structured clinical trial. These trials involve groups of diverse people and measure how much a vaccine reduces their risk of getting sick. If a vaccine has high efficacy, that means there were significantly fewer people in the vaccinated group who got sick compared with the unvaccinated group. 
  • Vaccine effectiveness measures successful immune protection under real-world conditions. If a vaccine has high effectiveness, there is clear evidence that the vaccine protects the public against severe infection, symptomatic illness, hospitalization, and death. 

Measuring vaccine effectiveness is a complex process. Oftentimes vaccine effectiveness data are not identical to vaccine efficacy data. This is because vaccine effectiveness takes into account the real world conditions in which vaccines are actually administered. Without context these statistics can oversimplify the number of factors used to determine outcomes. Vaccine effectiveness varies based on location, population, and specific health outcomes. The CDC and other public health researchers routinely monitor and evaluate data for vaccine effectiveness.

Added December 14, 2023 

It’s true that COVID-19 vaccines were developed more quickly than most previous vaccines, but their development built upon many prior decades of work.

The process included the same rigorous safety reviews that are required for all new vaccines. Researchers were able to develop the vaccines quickly, safely, and effectively because the urgency of the pandemic created greater access to research funding, reduced bureaucratic obstacles, and encouraged unparalleled levels of government and industry cooperation.

As new variants appear, vaccine researchers will continue to update COVID-19 vaccines to provide the highest level of protection against the virus.

Updated December 14, 2023 

In general, you are considered “fully vaccinated” and up-to-date with COVID-19 guidelines after you have received the recommended doses of the updated vaccine for your age, health status, and vaccine history. Depending on these factors, being “fully vaccinated” can look different for each person. As science and the virus evolve, so does our understanding of what it means to be “fully vaccinated”. Researchers continue to watch closely for signs of waning vaccine immunity over time, monitor new variants of the virus, and track how data differs across groups.

If you are not sure if you are “fully vaccinated” consult with a healthcare provider about completing the primary vaccine series, getting at least one updated Pfizer-BioNTech or Moderna dose, or getting the updated Novavax series. 

Updated December 14, 2023 

The risk of having a serious adverse reaction to the COVID-19 vaccine is very low—far lower than the risk of contracting COVID-19. 

Severe adverse reactions after vaccination are extremely rare, but can cause long-term health issues. Rare adverse events – such as anaphylaxis and other allergic reactions, blood-clotting syndromes, heart inflammation, autoimmune diseases impacting the nervous system, and death – have been identified and investigated through the Vaccine Adverse Event Reporting System (VAERS)

Common reactions to COVID-19 vaccination include mild side effects, such as limb soreness, fatigue, low-grade fever, headaches, and chills. These reactions typically resolve within a few days.

Updated December 14, 2023 

COVID-19 vaccination is essential for everyone, including people who have already had COVID-19. Data show that recently sick people who do not get vaccinated at least two months after their recovery are more likely to get COVID-19 again than people who get vaccinated after their recovery. This means that the COVID-19 vaccines increase protection against each COVID-19 re-infection.

Although highly contagious variants have led to breakthrough infections among vaccinated people, being unvaccinated is a clear health risk. Immunity provided by the updated COVID-19 vaccines is more safe, robust, and consistent, when compared to immunity from COVID-19 infection. COVID-19 vaccination is the best way to protect against death, hospitalization, and long-term COVID-19 health consequences.

Updated December 14, 2023 

Vaccines, including the COVID-19 vaccines, cannot provide 100% or permanent immunity. But COVID-19 vaccines are extremely effective in preventing serious illness, hospitalization, and death. Given that COVID-19 is an evolving virus, researchers continue to monitor how long vaccines provide immunity, which groups may benefit from additional doses, and how well the vaccines protect against new variants of the virus.

“Breakthrough” COVID-19 infections refer to infections in people who have fully completed the recommended vaccination schedule. This type of infection is not uncommon and can occur for multiple reasons. When vaccinated, breakthrough infections typically lead to mild symptoms that do not require hospitalization. People who are not vaccinated continue to account for the vast majority of severe cases, hospitalizations, and deaths from COVID-19. 

If you have questions about your risk of COVID-19, how to protect yourself, or the vaccines, consult with your healthcare provider.

Updated December 14, 2023 

Messaging Resources about Development, Safety, and Effectiveness

Updated Toolkit: Children and COVID-19 Vaccination

Toolkit: FDA Approval

Communications Tool: Building Bridges

CDC Foundation: Vaccine Resource Hub

Misinformation Alerts about Development, Safety, and Effectiveness

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