Are the vaccines safe?

Marianne Freiberger and Rachel Thomas

With all the questions in the news we asked experts Matt Keeling and Sam Moore, epidemiological modellers at Warwick University and members of the JUNIPER modelling consortium, whether the COVID-19 vaccines are safe. Here is what we learned.

Yes the approved vaccines are safe. If you balance the risks, then taking the vaccine is much safer than taking your chances with COVID-19.

Click here to see the entire COVID-19 vaccines FAQ.

The vaccines that have reported results so far, Pfizer/BioNTech, Moderna and Oxford/AstraZeneca, have all gone through the same safety trials that any new medicine or vaccine needs to go through for approval (more on the trials here). So far the regulators in the UK, Canada and USA have approved the Pfizer vaccine, with approval of other vaccines expected to be announced around the world in the coming months.

But what does that mean? Vaccine trials have two primary aims: to establish the efficacy, or effectiveness (more on this here) of a vaccine, and to establish its safety. The Pfizer/BioNTech vaccine was given to over 20,000 people, while a similar number got a placebo (saline solution). There was a limited difference between the number of serious adverse effects between the two groups in the trial. Therefore, we can safely assume that no severe adverse events seen in the trial were due to the vaccine.

It is of course possible that there might be very rare side effects to the vaccine — which only affect, say, 1 in 100,000 people — that have not been picked up by the vaccine trials. But what we have to do is balance the risks. This year in the UK about 11 in 10,000 people died of COVID-19 (based on UK government data). This number is over 100 times larger than the number of people we would expect to experience an adverse reaction that only affects 1 in 100,000 people.

Render of the 2019-nCoV virion.

For older people, the risk of dying from COVID-19 is significantly higher than 11 in 10,000. And even for young, fit and healthy people who catch COVID-19, the risk of developing long COVID, resulting in debilitating and long-lasting health problems, could be as high as 1 in a 100. That means that this year we can expect that as many as 26 in 10,000 people in the UK have caught COVID and developed long COVID (based on UK government data) – this is over 250 times more than the number of people we would expect to experience an adverse reaction that only affects 1 in 100,000 people.

So, as we said above, if you balance the risks then taking the vaccine is much safer than taking your chances with COVID-19. If you are aiming to make a rational decision based on available evidence, then these calculations show that taking the jab is a no brainer.

It's also important to realise that the effects of the vaccines will continue to be strictly and systematically monitored once the vaccines have been rolled out. In the UK the Medicines and Healthcare products Regulatory Agency (MHRA) who is responsible for approving vaccines continue to monitor for adverse side effects, and will update their advice on the use of the vaccine as necessary.

If you balance the risks, then taking the vaccine is much safer than taking your chances with COVID-19.

You might be concerned by the news that two individuals recently suffered an allergic reaction to the Pfizer vaccine – but it's important to recognise these two people were not typical members of the population, they were one of the small number of people who, unfortunately, are at such a high risk of a severe allergic reactions that they carried an epipen at all times. There is always a risk of such allergic reactions to any vaccines but this risk is very small — about 1 in 900,000 for all the vaccines that are given in the UK. This is why people are asked to wait and be observed by the medical professional for a short period of time after they have been vaccinated.

And finally, this discussion has only considered the personal risks for someone considering being vaccinated. If you were to catch COVID-19 then the risk wouldn't be only to yourself but also to those close to you and people in the wider community. Vaccination programmes are implemented to protect whole communities from disease, including those who aren't able to be vaccinated for valid medical reasons. We hope that this form of altruistic protection will hold for the COVID vaccines, but it is still too early to tell if they stop transmission as well as disease (see more here).

Return to our COVID-19 vaccines FAQ

About this article


Matt Keeling

Matt Keeling is a professor at the University of Warwick, and holds a joint position in Mathematics and Life Sciences. He is the current director of the Zeeman Institute for Systems Biology and Infectious Disease Epidemiology Research (SBIDER). He has been part of the SPI-M modelling group since 2009.

Sam Moore is a postdoctoral research associate who has been working on vaccination modelling for Covid-19 after joining SBIDER within the University of Warwick at the start of the pandemic earlier this year.


Sam Moore

Both are members of JUNIPER, the Joint UNIversity Pandemic and Epidemic Response modelling consortium. It comprises academics from seven UK universities who are using a range of mathematical and statistical techniques to address pressing question about the control of COVID. The universities are Cambridge, Warwick, Bristol, Exeter, Oxford, Manchester, and Lancaster. You can see more content produced with JUNIPER here.

Marianne Freiberger and Rachel Thomas are Editors of Plus.

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