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COVID-19 Vaccines safety

Updated 20 September 2021 | Q&A

Information about COVID-19 vaccines is quickly evolving and WHO will aim to update this Q&A on an ongoing basis, as we learn more.

How do we know that COVID-19 vaccines are safe?

There are strict protections in place to help ensure the safety of all COVID-19 vaccines. Before receiving validation from WHO and national regulatory agencies, 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, and through that process to assure they remain safe for use around the world.


What are the side effects of COVID-19 vaccines?

Like any vaccine, COVID-19 vaccines can cause mild, short term side effects, such as a low-grade fever or pain or redness at the injection site. Most reactions to vaccines are mild and go away within a few days on their own. More serious or long-lasting side effects to vaccines are possible but extremely rare. Vaccines are continually monitored for as long as they are in use, to detect rare adverse events and implement approaches to limit their occurrence. 

Reported side effects to COVID-19 vaccines have mostly been mild to moderate and short-lasting. They include: fever, fatigue, headache, muscle pain, chills, diarrhoea, and pain at the injection site. The chances of any of these side effects following vaccination differ according to the specific COVID-19 vaccine.

In most cases, these can be managed with rest, plenty of non-alcoholic liquids, and paracetamol/acetaminophen for the typical side effects. Contact your care provider if the tenderness (pain) where you got the injection increases after 24 hours, or the side effects do not go away within a couple days. If you have difficulty breathing, chest pain, confusion, loss of speech or mobility, call a healthcare provider immediately. 


What is the link between COVID-19 vaccines and allergic reactions?

Severe allergic reactions have occurred rarely to some of the COVID vaccines. A severe allergic reaction – such as anaphylaxis – is a potential but rare side effect with any vaccine. In persons with a known risk, such as previous experience of an allergic reaction to a previous dose of the vaccine or any of the known components in the vaccine, precautions may need to be taken.

WHO recommends that healthcare providers assess the risk for severe allergic reactions prior to giving a COVID-19 vaccine by inquiring about previous reactions or known allergies to any components in the vaccine.  All immunization providers should be trained to recognize severe allergic reactions and take steps to treat such reactions if they occur. 

COVID-19 vaccine use is being closely monitored by national authorities and international bodies, including WHO, to detect serious side effects, including any unexpected reactions. This is helping us better understand and manage the specific risks of allergic reactions or other serious side effects to COVID-19 vaccines that may not have been detected during clinical trials, ensuring safe vaccination for all.


Should I take a blood thinner a few days before taking the vaccine, given reports of blood clotting caused by the vaccination?

No, you should not take a blood thinner unless your healthcare provider has prescribed that medication to treat an existing health condition. Covid-19 vaccines are safe for people taking blood thinners but you should let the person giving you the vaccine know about any medication you are taking BEFORE you are given the vaccine. 


What happens if an adverse event is reported?

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 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.

In the very rare cases where an adverse reaction is suspected to be related to the vaccine itself, the vaccine may be suspended from use while the situation is assessed. Further investigations will take place to determine what exactly caused the event, and corrective measures will be put in place. WHO works with vaccine manufacturers, health officials, researchers, and other partners to monitor any safety concerns and potential side effects on an ongoing basis.


Under what circumstances should a COVID-19 vaccine be recalled?

Vaccine recalls or withdrawals due to safety issues are rare. Recalls are usually initiated voluntarily by a vaccine manufacturer before any adverse events are reported. For example, ongoing monitoring of vaccine production may show that an irregularity has caused a batch of vaccines to lose their strength. In this case, people who have received a vaccine from that batch may need to be vaccinated again to ensure they are protected.


How will WHO inform the public about suspected or confirmed adverse events related to COVID-19 vaccines?

Suspected safety events officially reported to WHO go through a series of rapid verification steps involving an independent panel of experts. WHO shares the results of these evaluations on its website.

WHO also coordinates with local, regional, and national health officials to investigate vaccine safety concerns and advise on next steps. Information is also made available through the Vaccine Safety Net, a publicly available network of digital international resources on vaccine safety that have been approved by WHO.


Is it possible that someone vaccinated against COVID-19 will still get infected?

While COVID-19 vaccines have high levels of efficacy, especially against hospitalization and severe disease, no vaccine is 100% protective. As a result, there will be some small percentage of vaccinated people who fall ill with COVID-19 in spite of being vaccinated.

In addition to a vaccine's specific characteristics, several factors such as a person's age, their underlying health conditions, previous COVID-19 disease, current exposure to SARS-CoV-2, or the circulation of virus variants may have an impact on a vaccine’s effectiveness. We do not yet know how long immunity from different COVID-19 vaccines will last. That is one reason why, even as COVID-19 vaccines are being rolled out, we must continue using all public health measures that work to decrease exposure risk, such as physical distancing, masks, and handwashing.

For the first 14 days after getting vaccinated, you do not have significant levels of protection as the protection increases gradually. For a single dose vaccine, protection is generally considered to occur by two weeks after vaccination. For two-dose vaccines, both doses are needed to achieve the highest level of immunity possible.

While a COVID-19 vaccine is most effective against serious illness and death, we are still learning about their ability to protect you from getting infected and passing the virus on to others. To help keep yourself and others safe, and while vaccination rolling out in your community, continue to maintain at least a one metre distance from others, cover a cough or sneeze in your elbow, clean your hands frequently and wear a mask, particularly in enclosed, crowded or poorly ventilated spaces. Always follow guidance from local authorities based on the situation and risk where you live.


Who should be excluded from receiving COVID-19 vaccines?

Medical professionals can best advise individuals on whether or not you should receive a COVID-19 vaccine. There are very few conditions that would exclude someone from being vaccinated. Based on available evidence, people with a history of severe allergic reactions to any ingredients of the COVID-19 vaccine should generally be excluded from COVID-19 vaccination in order to avoid possible adverse effects.

If you are currently sick or experiencing symptoms of COVID-19, you can get vaccinated once your primary symptoms have resolved.

In addition to the general recommendations above, each vaccine may have specific considerations for specific populations and health conditions.


Is it safe for pregnant women, those planning to become pregnant, and breastfeeding mothers to receive COVID-19 vaccines?

In the interim, WHO recommends that for pregnant women the use of the COVID-19 vaccine be considered on the basis of a benefit vs risk assessment. To help pregnant women make this assessment, they should be provided with information about the risks of COVID-19 in pregnancy, the likely benefits of vaccination in the local epidemiological context, and the current limitations of safety data in pregnant women. WHO does not recommend pregnancy testing prior to vaccination. WHO does not recommend delaying pregnancy or terminating pregnancy because of vaccination.

Vaccination with any of the vaccine products that have been assessed by the WHO Strategic Advisory Group of Experts on Immunization can be offered to breastfeeding women. WHO does not recommend discontinuation of breastfeeding after vaccination. More evidence will continue to be sought in order to further inform WHO’s policy recommendations on this subject.


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)[1].

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).

Click here to see an infographic on myocarditis and pericarditis.


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[3]. 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 [4]. 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.

For more information, read the Statement of the COVID-19 subcommittee of the WHO Global Advisory Committee on Vaccine Safety (GACVS) COVID-19 subcommittee on reports of Guillain-Barré.


How will WHO ensure vaccine safety monitoring and response when COVID-19 vaccines are used?

Vaccine safety monitoring is ensured at the national, regional, and global level. As is standard practice in all national immunization programmes, WHO supports the set up of safety monitoring systems for COVID-19 vaccines in every country. After a COVID-19 vaccine is introduced in a country, WHO works with vaccine manufacturers, health officials and other partners to monitor for any safety concerns on an ongoing basis. Specific safety concerns that may arise are 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.


[1]  US Vaccine Adverse Events Reporting System (VAERS) as of June 11 2021, this is in the updated guidance from GACVS 

[2] 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)

[3] 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

[4] https://www.who.int/news/item/26-07-2021-statement-of-the-who-gacvs-covid-19-subcommittee-on-gbs