Updated 21st December 2021

COVID-19 Booster Jabs: What Are the After Effects?

Getting boosted now is the best way to protect yourself and those around you from COVID-19 through the winter. 

Millions of people have already received booster vaccines, with millions more to come over the next few weeks. Here’s what we know so far about the after effects of booster jabs and what you can expect when you get yours.

How does the booster vaccine make you feel?

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

We’ve seen through studying the effects of the first two shots that 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; and diarrhea or 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.

Together with our research colleagues at King’s College London, we looked at data from more than 317,000 ZOE COVID Study app contributors who logged at least one daily health report after their booster dose by the 23rd November 2021. 27,761 received the Moderna vaccine as their booster, while 289,250 got a Pfizer jab.

Overall, we found that 50,339 contributors (15.9%) reported having at least one whole body (systemic) effect, most commonly fatigue and headache. A much larger number of people - 232,596 (73.4%) - reported local effects like tenderness and pain around the site of injection.

Do booster vaccines feel worse?

The proportion of people experiencing whole body (systemic) effects after their booster is roughly the same (i.e low) compared to what we found for people having their first Pfizer doses (12.5%), compared with around one in three people having their first AstraZeneca vaccine (33%).

The proportion of people experiencing local effects like tenderness and pain where they were injected was similar to what we found for the first two doses.

Is it OK to ‘mix and match’ different COVID vaccines?

In the UK, most people are being offered either Pfizer or Moderna mRNA vaccine boosters, regardless of whether they had Pfizer, AstraZeneca or Moderna for their first two doses. AstraZeneca is only being offered as a booster for people who are unable to have an mRNA vaccine for medical reasons.

In this latest analysis, 134,637 contributors got the same booster as their previous two vaccine doses, while 182,374 people got a ‘mix and match’ of various vaccine types. 

For contributors who got three Pfizer doses, slightly fewer people reported systemic after effects after their booster (13.1%) compared to after their second dose (19.2%), and were also less likely to have local effects third time around.

By contrast, people getting ‘mix and match’ vaccines were 1.5 times more likely to have systemic after effects after their booster, particularly if they had a Moderna booster after two Pfizer or AstraZeneca vaccines.

Mix Match Vaccines

How well do boosters work?

To find out how well booster doses work, we compared data from ZOE COVID Study app contributors over the age of 55 who had received a booster with people in the same age group who hadn’t yet had their booster dose by 23rd November.

We calculate that three Pfizer jabs are 95% effective against symptoms of the Delta variants compared with being unvaccinated, while having a Pfizer booster after two doses of AstraZeneca is 91% effective against the Delta variant. 

Having a Moderna booster after AstraZeneca is 89% effective against Delta, and 92.5% effective following two Pfizer jabs. All the booster jabs protected very well against severe disease and hospitalization from Delta 

These findings only relate to the Delta variant, which was the predominant viral strain earlier in the Autumn. 

We don’t yet have enough data to know how well boosters protect against the Omicron variant or its severity, but we’re continuing to monitor the situation and we’ll be sharing our findings with you as soon as we can.

Can you still catch COVID-19 after being vaccinated?

While our app data and many other studies show that vaccines and boosters are safe and highly effective at reducing severe symptoms, hospitalizations and deaths from COVID-19, you can still catch and spread the virus after being vaccinated. And we’ve already heard reports on the ZOE app that many people have been infected with the Omicron variant even after being boosted. So far, all the reports appear to be mild disease.

It’s still really important to take sensible precautions to stop the virus spreading including ventilating rooms where people are gathering, wearing a mask, and taking regular lateral flow tests a few hours before meeting others. 

We’re working as fast as we can to find out more about the symptoms and severity of Omicron, track its spread through the UK, and measure the effectiveness of vaccines and boosters against the new variant. 

You can help by downloading the ZOE app and logging daily health reports, as well as tests and vaccines. It only takes a minute, but you’ll be contributing to vital research to help us build a clearer picture of what’s going on.

Stay safe and keep logging.