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

February 4, 2021

Millions of people in the UK have received the Pfizer COVID jab Here’s what we know so far about the vaccine and its after-effects.

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. These whole body (systemic) effects include headaches, fever, chills or shivers, tiredness (fatigue), muscle or joint pains, diarrhoea and feeling sick (nausea). 

It’s also common to experience local effects like 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.

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

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. 

So far, more than a million people have logged their vaccinations with ZOE. Our data team has analysed reports from 282,103 people who have received their first dose of the Pfizer vaccine, and 28,207 who’ve had both.

We found:

  • Around one in eight people who received one dose of the vaccine experienced at least one systemic effect within seven days of their jab, compared with roughly one in three getting the AstraZeneca jab
  • Most symptoms happen in the first two days after vaccination, with headache, fatigue and chills or shivers being the most common
  • Almost one in five (19.9%) of the participants under 55 years old were likely to experience after-effects, compared to around one in ten (9.6%) over 55s
  • Around seven in ten people who received their first dose had at least one local after-effect in their arm, most commonly pain and swelling in the day or two after the jab

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 7 out of 10 reporting local symptoms like pain and swelling.


Line graph showing rate of after-effects experienced after 1st and 2nd doses of the Pfizer COVID vaccine. Credit: ZOE


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

Our research shows that people who had previously experienced COVID-19 were more likely to experience the systemic (whole body) effects after vaccination. 

Around a third of people who had COVID-19 in the past reported at least one systemic effect within seven days of getting a Pfizer vaccine, compared with one in five people who hadn’t 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 rates of after-effects in people with previous COVID-19 infection and those without previous infection. Credit: ZOE


Is the Pfizer vaccine working?

While the full course of the Pfizer vaccine is two shots up to 12 weeks apart, the good news is that it seems to provide some degree of protection after just the first dose.

Our data shows a 58% reduction in infection rates between 12 to 21 days after the first dose of the Pfizer vaccine, improving to a 69% reduction after 21 days. 

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.

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 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. 

Want to be part of the largest independent community-led COVID vaccine study in the world? Log your jab with ZOE

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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