English version is updated on 29 November 2021 - This page answers the most frequently asked questions about COVID-19 vaccine safety. If the information you are looking for is not here, check out our related links on the right-hand side of the page.
How do we know that COVID-19 vaccines are safe?
There are strict precautions in place to help ensure the safety of all COVID-19 vaccines. Before receiving validation from WHO and national regulatory agencies for emergency use, COVID-19 vaccines must undergo rigorous testing in clinical trials to prove that they meet internationally agreed benchmarks for safety and efficacy.
Unprecedented scientific collaborations have allowed COVID-19 vaccine research, development, and authorizations to be completed in record time – to meet the urgent need for these vaccines while maintaining high safety standards. As with all vaccines, WHO and regulatory authorities will continuously monitor the use of COVID-19 vaccines to identify and respond to any safety issues that might arise. Through that process, we ascertain that they remain safe for use around the world.
How does WHO ensure that COVID-19 vaccines are safe and effective?
The vaccines must be proven safe and effective in large Phase III clinical trials. Some COVID-19 vaccine candidates have already completed their Phase III trials and are being used globally, and many other vaccine candidates are being developed and are still in the trial phase.
Independent reviews of the efficacy and safety evidence are required by WHO for each vaccine candidate, including regulatory review and approval in the country where the vaccine is manufactured, before WHO considers it for emergency use listing.
An external panel of experts convened by WHO (the Strategic Advisory Group of Experts on Immunization (SAGE), analyses the results from clinical trials, along with evidence on the disease, age groups affected, risk factors for disease, programmatic use and other information. SAGE then recommends whether and how the vaccines should be used. In addition to reviewing data for regulatory purposes, the evidence must also be reviewed for the purpose of policy recommendations on how the vaccines should be used.
The WHO’s Global Advisory Committee on Vaccine Safety (GACVS) monitors how approved vaccines behave in the real world and to identify any signals of adverse events following immunization. GACVS is an independent group of experts providing authoritative, scientific advice to WHO on vaccine safety issues of global or regional concern.
What systems are in place to monitor vaccine safety?
Vaccine safety monitoring is critical at national, regional, and global levels. After any vaccine is introduced, WHO works with vaccine manufacturers, health officials, national advisory committees and other partners to monitor for any safety concerns on an ongoing basis. Specific safety concerns that arise are then evaluated by WHO and an independent group of experts (the Global Advisory Committee on Vaccine Safety, or GACVS) in conjunction with the relevant national authorities.
As is standard practice in all national immunization programmes, WHO is supporting the implementation of safety monitoring systems for COVID-19 vaccines in every country.
How will WHO inform the public about suspected or confirmed serious adverse events related to COVID-19 vaccines?
Suspected safety events officially reported to WHO go through a series of rapid verification steps involving the Global Advisory Committee on Vaccine Safety (GACVS). WHO then shares the results of these evaluations on its website and publishes the results of any assessments.
WHO also coordinates with local, regional, and national health officials to investigate vaccine safety concerns and advise on next steps.
Additional vaccine safety information is also made available through the Vaccine Safety Net, a global network of websites providing reliable information on vaccine safety that have been verified by WHO.
Under what circumstances should a COVID-19 vaccine be recalled?
In rare situations where a serious adverse reaction is suspected to be related to the vaccine itself, the vaccine may be temporarily suspended from use while the situation is assessed. Investigations will take place to determine what exactly caused the event, and corrective measures will be put in place. WHO works closely with vaccine manufacturers, health officials, researchers, and other partners to monitor any safety concerns and potential side effects.
Vaccine recalls or withdrawals due to safety issues are rare. Recalls are generally associated with problems identified during the monitoring of batches of vaccines through quality control systems, stability studies and reports from the field, including cold chain issues where some vaccines have not been stored at the right conditions and so are no longer safe or effective. In this case, people who have received a vaccine from that batch may need to be vaccinated again to ensure they are protected. This is why vaccines are so closely monitored – to ensure that any issues with their production, storage or use can be rapidly identified and resolved.
What are the side effects of COVID-19 vaccines?
Like with any vaccine, some people will experience mild to moderate side effects after being vaccinated against COVID-19. This is a normal sign that the body is developing protection. Side effects to COVID-19 vaccines include a fever, tiredness, headache, muscle ache, chills, diarrhoea and pain or redness at the injection site. Not everyone will experience side effects. Most side effects go away within a few days on their own. You can manage any side effects with rest, plenty of non-alcoholic liquids and taking medication to manage pain and fever, if needed.
If you are worried that the side effects that you are experiencing are unusual, if the pain in the arm where you got the injection gets worse after 24 hours or your side effects don’t go away in a few days, contact your healthcare provider for advice.
More serious or long-lasting side effects to COVID-19 vaccines are possible but extremely rare. If you experience difficulty breathing, chest pain, confusion, loss of speech or mobility after your vaccine, contact your healthcare provider immediately. Vaccines are continually monitored for as long as they are in use to detect and respond to rare adverse events.
Does having side effects mean that the vaccine is working? What does having no side effects mean?
The vaccine stimulates your immune system to protect you from the virus. This process can sometimes cause side effects like fever, chills or headache, but not everyone will experiences any side effect. The presence or magnitude of the reaction you may have vaccination does not predict or reflect your immune response to the vaccine.
You do not have to have side effects in order to be protected.
What should I do if I think my side effects are unusual?
If you are worried about your side effects, contact your healthcare provider and let them know about your recent vaccination.
Is there a link between COVID-19 vaccines and allergic reactions?
In very rare cases, some people may experience an allergic reaction after being vaccinated against COVID-19. A severe allergic reaction – such as anaphylaxis – is a very rare side effect of any vaccine.
If you have a history of allergic reactions, talk to your healthcare provider before you get vaccinated. They will be able to give you advice. In some cases, precautions will need to be taken for people who have known allergies to previous doses of the vaccine or known components of the vaccine.
Healthcare workers administering COVID-19 vaccines should be trained to recognise and treat serious allergic reactions. This is why people being vaccinated against COVID-19 will be asked to stay at the vaccination site for a period of time following their injection, to ensure that anyone experiencing an allergic reaction can receive prompt treatment.
Reports of adverse events following COVID-19 vaccination (including allergic reactions) are closely monitored by national authorities and international bodies, including WHO for the early detection of serious side effects.
What happens if a serious adverse event is reported after vaccination?
As with any vaccine, it is essential to closely monitor the safety and effectiveness of COVID-19 vaccines that are used in immunization programmes. If a serious health problem is reported following vaccination, a thorough investigation should take place by the public health programme in the country.
It is rare to find that health problems occurring following receipt of a vaccine are actually caused by the vaccine itself. Health problems following vaccination are most often found to be coincidental and entirely unrelated to vaccination. Sometimes they are related to how the vaccine has been stored, transported, or administered. Errors related to the delivery of the vaccine can be prevented by better training health workers and strengthening supply chains.
The results of the investigation will then inform next steps. The safety of COVID-19 vaccines is the top priority of the World Health Organization.
Find out more about how the safety of vaccines is monitored.
Is it safe to mix-and-match different COVID-19 vaccines?
If you can, get your second or third dose with the same vaccine you received for your first dose. There is not much data available yet on mixing different vaccines.
In situations where a second dose of AstraZeneca is not available, WHO's Strategic Advisory Group of Experts (SAGE) has stated that a second dose of Pfizer or Moderna can be used. There is some evidence that people who receive one dose of AstraZeneca followed by a dose of Pfizer or Moderna have a higher likelihood of mild side effects.
People over 60 who have received two doses of Sinovac and Sinopharm can also be given a third dose to help boost their immunity, once a high level of coverage of the priority groups has been achieved. SAGE has stated that Pfizer or AstraZeneca can be used for the third dose if the original vaccine is not available.
Further trials are underway to understand more about mixed doses, which will inform any future changes to WHO’s recommendations.
Should women who are on their periods take the COVID-19 vaccine?
Women can receive a vaccine at any point in their menstrual cycle.
Are mRNA vaccines safe? If they’re based on new technology, how can we be sure?
The COVID-19 mRNA vaccine technology has been rigorously assessed for safety, and clinical trials have shown that mRNA vaccines produce an immune response that has high efficacy against disease. mRNA vaccine technology has been studied for several decades, including in the contexts of Zika, rabies, and influenza vaccines. mRNA vaccines are not live virus vaccines and do not interfere with human DNA.
Is there a risk of heart inflammation (myocarditis or pericarditis) from mRNA COVID-19 vaccines?
Globally, mRNA vaccines such as Pfizer and Moderna have been used to protect millions of people against COVID-19. A significant amount of data is available from both clinical trials and country surveillance programmes on their efficacy and safety. Some mild side effects are expected after vaccination; these are a normal sign that the body is developing protection.
There have been reports of very rare cases of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the membrane surrounding the heart) following the second dose of mRNA COVID-19 vaccines. Myocarditis and pericarditis can be caused by many factors, including infections, viruses, medicines and environmental factors. The currently available data suggests that there is also a potential relationship between these symptoms and mRNA vaccines. Research is underway to understand more.
Cases have been mostly reported in younger men aged 12 – 29 (40.6 cases of myocarditis per million second doses) compared to females of the same age group (4.2 cases per million second doses).
The symptoms of myocarditis and pericarditis are generally mild. Fast treatment with medication and rest can help to avoid long term heart damage and death. If you experience new and persisting chest pain, shortness of breath or have a racing or pounding heartbeat within a few days of vaccination, contact your doctor immediately. The benefits of these vaccines greatly outweigh the risk of myocarditis and pericarditis by preventing deaths and hospitalisations due to COVID-19.
For more information, please read the updated guidance from the COVID-19 subcommittee of the WHO Global Advisory Committee on Vaccine Safety (GACVS).
Do the COVID-19 Astra Zeneca and Johnson & Johnson/Janssen vaccines cause blood clots?
Globally, COVID-19 vaccines such as AstraZeneca and Johnson &Johnson/Janssen have been used to protect millions of people. Data is available from both clinical trials and preliminary data from country surveillance programmes on their efficacy and safety. Some mild to moderate side effects such as fever, muscle and head aches, soreness around the injection site and tiredness are expected to affect some people after vaccination. These are a normal indications that the body is developing protection.
There have been reports of very rare but serious cases of blood clots accompanied by low platelet counts (known as thrombosis with thrombocytopenia syndrome (TTS)) occurring 3 to 30 days after vaccination with COVID-19 non-replicant adenovirus vector-based vaccines (such as the AstraZeneca and Janssen vaccines). Read more about the different types of COVID-19 vaccines.
With the AstraZeneca vaccine, as of 15 July 2021, the data shows that these symptoms occur in about four to six people out of every million vaccinated (this figure varies based on age, sex and geographical location)[2}. Younger adults appear to be at higher risk than older adults. More research is underway to understand more about how people may be differently affected.
With the Janssen vaccine, as of the 7 May 2021, the US Food and Drug Administration and the Centers for Disease Control and Prevention had reviewed 28 reports of TTS out of a total of more than eight million people vaccinated. It is possible that that there a causal link between the vaccine and these symptoms, but more data is needed.
TTS is very rare; however, blood clotting is a common health problem caused by many factors. Not all clots that occur after vaccination with Astra Zeneca or the Janssen vaccine will be due to TTS. The risk of blood clots is far higher from COVID-19 itself than from either vaccine.
If you experience a new, severe, persistent headache, blurred vision, chest pain, severe abdominal pain, leg swelling or unusual skin bruising and shortness of breath between three to 30 days following vaccination, contact your doctor immediately. WHO has published interim guidance for clinical case management of TTS following vaccination.
The benefits of the COVID-19 Astra Zeneca and Janssen vaccines are far greater than the very small risk of TTS. As well as protecting you from severe disease and death due to COVID-19, being vaccinated can protect you against complications from ‘long COVID’, provide some protection for your close contacts and community by reducing transmission, and can reduce the risk of severe disease from some variant strains. WHO recommends that these vaccines continue to be used to protect priority groups.
For more information, read the Statement of the COVID-19 subcommittee of the WHO Global Advisory Committee on Vaccine Safety (GACVS) on safety signals related to the Johnson & Johnson/Janssen COVID-19 vaccine and the GACVS review of latest evidence of rare adverse blood coagulation events with AstraZeneca COVID-19 Vaccine.
Do the AstraZeneca and Johnson & Johnson/Janssen COVID-19 vaccines cause Guillain-Barré syndrome?
Globally, vaccines such as AstraZeneca and Johnson &Johnson/Janssen have been used to protect millions of people against COVID-19. Data is available from both clinical trials and preliminary data from country surveillance programmes on their efficacy and safety. Some mild to moderate side effects such as fever, muscle and head aches, soreness around the injection site and tiredness are expected to affect some people after vaccination. These are a normal indications that the body is developing protection.
There have been very rare reports of cases of Guillain-Barré syndrome occurring within 42 days of vaccination with the AstraZeneca and Janssen vaccines (adenovirus vector vaccines). Guillain-Barré syndrome is a rare condition in which a person’s immune system attacks the nerves, causing muscle weakness, tingling and/or loss of sensation in the arms and/or legs. Most people affected recover fully. Although these cases happened after vaccination, it is not yet possible to tell whether they were related to or caused by the vaccines, or if they were coincidental. The benefits of vaccination against COVID-19 greatly outweigh any possible risk.
As of the 27 June, the European Medicines Agency had received reports of 227 cases of Guillain-Barré syndrome in the European Union following vaccination with AstraZeneca. Around 51.4 million doses of Vaxzevria (the brand of AstraZeneca produced in Europe) had been administered within the European Union by 20 June 2021. For the Janssen vaccine, the US Advisory Committee on Immunization Practices reported that 100 cases of Guillain-Barré syndrome had been reported through the Vaccine Adverse Events Reporting System in the United States of America (USA) as of 30 June, with approximately 12.2 million doses of the Janssen vaccine administered. Outside of the European Union and the USA, cases of Guillain-Barré syndrome have also been reported; however further investigation is needed to determine whether the rate has increased from usual levels.
Guillain-Barré syndrome has many causes, such as bacterial or viral infections, surgery or vaccine administration. It can also be caused by COVID-19. In 2011, it was estimated that the incidence of Guillain-Barré syndrome in Europe and Northern America was 0.8 to 1.9 cases out of every 100 000 people . It can affect people of all ages but is more common in adults and in males. Most people recover fully even from the most severe cases of Guillain-Barré syndrome; however, it is potentially life threatening and some people affected may need intensive care.
If you experience any of the following symptoms within 42 days of vaccination, contact your healthcare provider immediately:
- Difficulty walking
- Difficulty with facial movements, including breathing, speaking, chewing or swallowing
- Double vision or being unable to move your eyes
- Difficulty with bladder or bowel control
- Weakness or tingling sensations in the legs, arms and face
The benefits of the COVID-19 Janssen and AstraZeneca vaccines are far greater than the very small risk of Guillain-Barré syndrome. As well as protecting you from severe disease and death due to COVID-19, being vaccinated can protect you against complications from ‘long COVID’, provide some protection for your close contacts and community by reducing transmission, and can reduce the risk of severe disease from variant strains. COVID-19 disease can also cause Guillain-Barré syndrome. WHO recommends that these vaccines continue to be used to protect priority groups.
 US Vaccine Adverse Events Reporting System (VAERS) as of June 11 2021, this is in the updated guidance from GACVS
 Global Advisory Committee on Vaccine Safety (GACVS) review of latest evidence of rare adverse blood coagulation events with AstraZeneca COVID-19 Vaccine (Vaxzevria and Covishield)
 Statement of the COVID-19 subcommittee of the WHO Global Advisory Committee on Vaccine Safety (GACVS) on safety signals related to the Johnson & Johnson/Janssen COVID-19 vaccine