Frequently Asked Questions about Development, safety, and effectiveness

On January 13, the Centers for Disease Control and Prevention (CDC) announced that a vaccine safety monitoring system called the Vaccine Safety Datalink picked up a signal possibly linking Pfizer’s COVID-19 bivalent vaccine with an increased risk of stroke in people 65 and older. Here’s what you need to know:

  • What’s a safety signal and what does it mean? A “safety signal” occurs when vaccine monitoring systems pick up on an adverse medical event after vaccination at a rate higher than statistically expected. When a system “signals” a possible adverse event, researchers review other monitoring systems to check if other data are signaling the same risk. The Vaccine Safety Datalink, a collaborative database involving the CDC and healthcare organizations, includes electronic health records on 12 million people, and it is one of several independent vaccine safety monitoring systems. To date, no such possible risk of stroke among people age 65 and older has been detected by these other monitoring systems.
  • After extensive review of the latest vaccine safety data, federal health officials have said it’s very unlikely that there’s a true clinical risk of stroke associated with Pfizer’s COVID-19 booster. After the possible risk was detected in the Vaccine Safety Datalink, no other systems independently monitoring COVID-19 vaccine safety have observed any correlation between Pfizer’s updated bivalent vaccine and an increased risk of stroke. This safety signal has not been seen with Moderna’s bivalent COVID-19 vaccine.
  • The safety system that monitors COVID-19 vaccine safety is the most extensive in U.S. history. According to safety experts, safety signals occur frequently, which is a sign that the safety system monitoring COVID-19 vaccines is effective and sensitive enough to detect potential concerns and safety risks. The CDC will continue to monitor vaccine safety systems for any updates.
  • The COVID-19 vaccines and updated boosters are safe. Getting vaccinated and boosted remains our best defense against serious illness and hospitalization due to COVID-19. The CDC continues to recommend everyone age 6 months and older stay up to date with COVID-19 vaccination, including those who are eligible for an updated bivalent booster. As the virus continues to evolve and new variants emerge, the data consistently show that COVID-19 vaccines have saved tens of millions of lives, are safe, and continue to be effective at preventing severe illness. 

For more messaging guidance on COVID-19 vaccine safety, see our Talking Points on Vaccine Safety and Effectiveness

Updated on January 19, 2023 

There are a few reasons why people who are vaccinated continue to get COVID-19. For one, no vaccine is 100% effective at preventing infection, and highly contagious variants have led to breakthrough infections among vaccinated people. Also, the level of protection from the vaccine decreases over time, leading to less protection against the virus. Ultimately, as the total number of vaccinated people increases, the pool of unvaccinated people gets smaller—that means proportionally, more cases will be among the vaccinated.

While vaccines are developed in part to prevent infection from disease, the main goal of vaccines is to prevent severe illness or death. The COVID-19 vaccines continue to be highly effective in reducing risk of severe disease, hospitalization, and death, and can provide sustained protection when you receive a booster dose. When COVID-19 cases rise, breakthrough infections among vaccinated individuals are significantly more likely to be mild cases, while unvaccinated people are more likely to become severely ill or require hospitalization. CDC data show that through December 25, 2021, the risk of being hospitalized with COVID-19 in the U.S. was 16 times greater for unvaccinated adults than fully vaccinated adults.

Updated February 18, 2022 

The COVID-19 vaccines have received the most intense safety monitoring in U.S. history, which has allowed public health officials to make science-based recommendations that keep people safe.

All COVID-19 vaccines have been rigorously tested and reviewed. The vaccine’s clinical trials three-phase process was detailed and thorough, and no shortcuts were taken. More than 150,000 people participated in U.S. clinical trials of the vaccines, and now, hundreds of millions of vaccine doses in the U.S. have been safely administered. Data from trial will continue to be collected for two years after each vaccine is first administered to ensure that they are safe for the long term. As with all vaccines, there will be ongoing monitoring for adverse events among people who are vaccinated into the future.

Updated October 12, 2021 

It may seem like the vaccines were developed quickly, but the process included rigorous safety reviews required for all new vaccines. The urgency of the pandemic created greater access to research funding, reduced bureaucratic obstacles, and encouraged unparalleled levels of government and industry cooperation. With these supports in place, scientists built upon previous work on coronavirus vaccines and on mRNA vaccine technology to develop these new vaccines quickly and effectively.

Updated October 12, 2021 

In general, you are considered fully vaccinated for COVID-19 two weeks after you have received the second dose in a two-dose series (Pfizer-BioNTech or Moderna) or two weeks after you have received a single-dose vaccine (Johnson & Johnson).

As the science and the virus evolve, so does our understanding of what it means to be fully vaccinated.  Scientists and medical experts continue to closely watch for signs of waning vaccine immunity over time, how well the vaccines protect against new variants of the virus, and how that data differs across the population.

While additional or booster doses are recommended for some people, the CDC definition of what it means to be “fully vaccinated” has not changed at this time. More messaging guidance about booster doses can be found here.

Updated October 12, 2021 

The CDC recommends all people age 5 and older get vaccinated against COVID-19, including people who were previously infected with the virus. Data show that immunity in people who have been infected with COVID-19 wanes over time, and scientists continue to study this. New data show that COVID-19 vaccination can provide a higher, more robust, and more consistent level of immunity to protect people from COVID-19 than antibodies from infection alone. 

COVID-19 vaccination is effective in preventing reinfection in people who previously had COVID-19. One study, for example, showed that among people hospitalized with COVID-19, those who were previously infected with COVID-19 were 5 times more likely to get COVID-19 again if they were unvaccinated than people who were fully vaccinated. For that reason, even if you have already had COVID-19, vaccination is an important step to protect yourself and those around you.

Updated November 15, 2021 

While COVID-19 vaccines are highly effective, no vaccine provides 100% immunity. Because this is a new virus, scientists and medical experts continue to monitor how long immunity lasts, whether some groups may need additional doses, and how well the vaccines protect against new variants of the virus.

Data continue to show that the COVID-19 vaccines are extremely effective in protecting fully vaccinated people from catching and spreading the virus, including the Delta variant, and scientists continue to monitor vaccine efficacy for new variants. A small percentage of vaccinated people experience breakthrough cases, but they are much more likely to have milder symptoms than unvaccinated people who get COVID-19.

Unvaccinated people continue to account for the vast majority of severe cases, hospitalizations, and deaths from COVID-19. CDC data show that in August 2021, the risk of dying from COVID-19 in the U.S. was more than 11 times greater for unvaccinated people than for fully vaccinated people.

The risk of severe illness from COVID-19 is elevated for some groups — including older adults, people with underlying medical conditions, immunocompromised people, and pregnant or recently pregnant women. If you have questions about your risk of COVID-19, how to protect yourself, or the vaccines, speak to your health care provider.

Updated December 9, 2021 

The threat of COVID-19 is real and urgent, and getting vaccinated is the best way to protect yourself. Side effects to the COVID-19 vaccines are typically mild and subside in one to two days — like soreness in the arm, fatigue, headaches, or a slight fever.

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. The CDC and FDA are closely monitoring vaccine outcomes to ensure safety.

If you have a question about the vaccines, talk with your healthcare provider.

Updated October 12, 2021 

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

Misleading headlines inaccurately suggest a link between COVID-19 booster and stroke

The CDC investigated a potential link between the Pfizer bivalent COVID-19 booster and increased stroke risk in older adults. The investigation found no increased stroke risk. Several news headlines noted the potential link but not the result of the investigation. As a result, vaccine opponents are falsely claiming that the CDC found a link. Recommendation:

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