An anti-vaccine website known for promoting the claim that vaccines cause autism is using a recent report to suggest that COVID-19 vaccines are causing a rise in so-called “turbo cancers.” Recommendation: Medium Risk Sign Up for Public Health Alerts email(Required) About Us The Public Health Communications Collaborative (PHCC) was formed in 2020 to coordinate and
Development, safety, and effectiveness
Frequently Asked Questions about Development, safety, and effectiveness
Yes, COVID-19 vaccines are safe. 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. Based on this thorough process, public health officials can make evidence-based recommendations to keep the public safe.
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 will continue to be collected for two years after each vaccine is first administered to ensure that they are safe over the long term. As with all vaccines, there will be ongoing monitoring for adverse events among people who are vaccinated in the future.
Yes, the COVID-19 vaccines are highly effective at reducing the risk of severe infection, symptomatic illness, hospitalization, and death by providing immune protection that lasts several months.
The original Moderna and Pfizer-BioNTech COVID-19 vaccines approved in December 2020 were both shown to have 95% efficacy in preventing symptomatic COVID-19 illness. These vaccines were also shown to have high efficacy, 93% and 88% efficacy respectively, in targeting the predominant strain and preventing hospitalization.
However, a vaccine can only protect against a strain that it recognizes. As the virus continues to circulate and mutate, new variants are less recognizable to our immune systems and may lead to new infections — even for people who are vaccinated. COVID-19 vaccine effectiveness wears off over time and the coronavirus is constantly evolving and mutating. As a result, updated vaccines have been developed to more closely target the currently circulating variants of concern and “boost” the public’s immune protection to avoid another wave of infections, hospitalizations, and deaths.
The updated Moderna and Pfizer-BioNTech vaccines that were approved in fall 2023 are critical in boosting the public’s COVID-19 immunity. According to the latest research data, the newly updated COVID-19 vaccines are effective at producing strong immune responses to new variants. The updated vaccines provide increased protection against currently circulating COVID-19 strains, 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 subvariants than before their re-vaccination.
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 real-world proof that the vaccine protects people against severe infection, symptomatic illness, hospitalization, and death.
While vaccine effectiveness can be quantified using percentages, without context these statistics can oversimplify the number of factors used to determine outcomes. Vaccine effectiveness varies based on location, population, research approach, and specific health outcomes. CDC and other public health researchers routinely monitor and evaluate data for vaccine effectiveness.
It’s true that these specific vaccines were developed more quickly than most, but their development built upon many decades of work on coronavirus vaccines and mRNA technology.
The process included the same rigorous safety reviews that are required for all new vaccines. Scientists 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 develop updated COVID-19 vaccines, also called “boosters,” to provide the highest level of protection against the virus.
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.
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.
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.
Common reactions to COVID-19 vaccination include mild side effects, such as limb soreness, fatigue, low-grade fever, headaches, and chills, which typically resolve within a few days.
Severe adverse reactions after vaccination are extremely rare, but can cause long-term health issues. Adverse events, such as anaphylaxis and other allergic reactions, blood-clotting syndromes, heart inflammation, autoimmune diseases impacting the nervous system, and death, have been reported within the Vaccine Adverse Event Reporting System (VAERS).
If you have a question about the risks associated with vaccines, talk with your healthcare provider.
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.
Given that COVID-19 is an evolving virus, researchers and medical experts 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.
COVID-19 vaccines, like all other vaccines, do not provide 100% immunity. But they have been shown to be extremely effective in preventing serious illness, and they provide continued protection during periods of peak respiratory virus spread, such as the fall and winter months.
“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. With the benefit of protection from the COVID-19 vaccine, breakthrough infections typically produce mild symptoms and 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.
COVID-19 vaccines are a very important tool to protect against serious health outcomes, however vaccination alone is not enough to protect the public. It is also encouraged to practice other precautionary measures such as wearing a mask, covering a sneeze with a bent elbow or tissue, handwashing, avoiding crowded and closed spaces, social distancing from others, and isolating when sick.
Those who have compromised immune systems particularly benefit from practicing all recommended health precautions, including being vaccinated. If you have questions about your risk of COVID-19, how to protect yourself, or the vaccines, consult with your healthcare provider.
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.
Messaging Resources about Development, safety, and effectiveness
Misinformation Alerts about Development, safety, and effectiveness
A false analysis that was originally published in September claims that COVID-19 vaccines have caused 17 million deaths. Several vaccine opponents are recirculating the report. Recommendation: Medium Risk Sign Up for Public Health Alerts email(Required) About Us The Public Health Communications Collaborative (PHCC) was formed in 2020 to coordinate and amplify public health messaging on
A widely-circulated social media post questions why a vaccine against COVID-19 was developed so quickly when there are no vaccines for cancer, HIV, or the common cold. Recommendation: Medium Risk Sign Up for Public Health Alerts email(Required) About Us The Public Health Communications Collaborative (PHCC) was formed in 2020 to coordinate and amplify public health