Here’s what we know so far about the after effects of the Pfizer COVID vaccine

February 4, 2021

The Pfizer/BioNTech COVID-19 vaccine began rollout in the UK on 8th December. Since then, millions of people have been vaccinated, including the elderly, care workers and frontline NHS staff. 

Since the roll out of the vaccines in the UK, we’ve been asking everyone who gets vaccinated to log their jab, so that we can understand more about the impact of the new COVID-19 vaccines and their effects. 

As well as asking people to log daily reports about their general health for a week after vaccination, which captures information about the impact of the vaccine on the whole body (systemic effects), we’re also asking about pain and swelling at the injection site (local effects). 

So far, more than 275,000 people have logged their vaccinations in the app, many of them healthcare workers. 

Our data team has analysed reports from nearly 40,000 people who were vaccinated in December: 23,308 people who had received just one dose, and 12,444 who had both. 

Here’s what we know so far.

How does the Pfizer COVID-19 vaccine make you feel?

COVID-19 vaccines work by using a harmless version or component of the SARS-CoV-2 coronavirus to train the immune system, so when we encounter the virus for real we’re able to fight it off. 

This ‘training’ response can feel a bit like the effects we get when we’re fighting off a real infection, including headaches, fever, chills or shivers, tiredness (fatigue), muscle or joint pains, diarrhoea and feeling sick (nausea). 

It’s also common to experience pain, swelling, redness or itchiness at the site of the injection, or swelling of the glands (lymph nodes) in the armpit.

While they may make you feel grotty, all these effects are a sign that your immune system is kicking into action to protect you from COVID-19. 

At the same time, don’t worry if you don’t experience any of these effects after your vaccine. Your immune system will still be learning to respond to the virus - it’s just not making a fuss about it.

How many people have after effects from the Pfizer COVID-19 vaccine?

We found that around one in seven people who received one dose of the vaccine experienced at least one systemic effect within seven days of their jab. 

We also found that around four in ten people who received their first dose had at least one after effect in their arm, most commonly pain and swelling in the day or two after the jab. 

Most symptoms happen in the first two days after vaccination, with headache, fatigue and chills or shivers being the most common

Does your second COVID-19 vaccine dose feel worse than the first?

After effects are more common the second time around, with around one in five who received their second dose of the Pfizer vaccine logging at least one systemic effect.

Similarly, more people experienced effects in their arm after their second dose, with about half reporting local symptoms like pain and swelling.

Line graph showing systemic after effects post-vaccination dose


Do you feel worse after vaccination if you’ve already had COVID-19?

Because many people have been continually using the ZOE COVID app throughout the pandemic, we were also able to compare the effects of vaccination in people who had already had COVID-19 with those who had not.

We found that previous exposure to coronavirus makes it more likely that someone will experience systemic effects after vaccination. 

Around a third of vaccinated people who had COVID-19 in the past reported at least one systemic effect within seven days from their vaccination, compared with just under one in five people who had not already had the virus. 

Around one in twenty people who had previously had COVID-19 experienced after effects for more than three days during the first week, and around one in fifty were affected for six days or more. 


Bar graph showing rate of people who experienced systemic effects post-vaccination

Can you still catch COVID-19 after being vaccinated?

You can still catch COVID-19 after being vaccinated as it takes time for your immunity to build up and no vaccine provides 100% protection. 

We also still don’t know for sure whether people who have been vaccinated can still pass on the coronavirus, even if they themselves don’t get ill from it. 

It’s important to make sure that you keep to local lockdown restrictions and follow the guidelines on social distancing, wearing face coverings, ventilation and hygiene to protect yourself and others around you, even once you’ve been vaccinated.

Remember that the after effects of vaccination can look a lot like COVID-19 symptoms. If you’re logging your symptoms in the app after getting your jab and are offered a COVID test, please take it.

You should get a test and self-isolate if you are still feeling unwell several days after your jab, especially if you lose your sense of smell (anosmia) or develop a persistent cough. 

Help us understand the effects and impact of COVID-19 vaccines by logging your jab

We need to gather as much data as we can about the impact and any after effects of COVID-19 vaccines to support the rollout and help end the pandemic. 
If you’re not already using it, download the ZOE COVID Symptom Study app and use it to record your COVID-19 vaccine, followed by daily health reports to help us monitor the after effects and impact of vaccination. You can also register a profile and log reports on behalf of relatives or people you are caring for who may not be able to use the app themselves. 

By logging your vaccination you’ll be part of the largest independent community-led COVID vaccine study in the world, providing vital information to ensure public safety and help end the pandemic.

Together we’ll get through this. Stay safe and keep logging.

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