Are COVID booster jabs safe?

November 15, 2021

Thanks to our contributors, we’ve recorded a quarter of a million COVID boosters and third primary doses through the ZOE COVID Study to better understand their safety and effectiveness. 

Have you logged your booster? Log in the app if you’ve not already done so, and read this guide to help you through.


What’s the difference between a booster and a third COVID jab?

Boosters: Boosters are being offered to all individuals who are considered in the older or more vulnerable category, who received vaccination in Phase 1 of the COVID-19 vaccination programme six months after completion of the primary vaccine course. At the moment in the UK, this includes those who are over 50, those who work in care homes, frontline health workers or adults with certain health conditions. If you’re eligible and have not received it yet please book yourself in! Pfizer is preferred for this even if you had a different primary dose, but some individuals may be offered a half dose (50µg) of the Moderna COVID-19 vaccine.

Third primary dose: a third primary dose is being offered to those who had severe immunosuppression at the time of receiving their first and second primary doses. A third dose aims to bring these individuals up nearer the level of immunity as the non-immunosuppressed achieve from two doses. The third primary dose should ideally be given at least 8 weeks after the second dose in the UK, or 28 days after the second dose in the USA. Moderna and Pfizer vaccines are preferred for this.

What’s the dosage of the booster?

The booster is the same dose as 1st and 2nd dose of Pfizer, Moderna will be half dose as it was stronger, if you’re immunocompromised and had the third primary dose, it’s the same dosage as the booster but you may be given it at a different time. 

Please log boosters and third jabs in the app as another COVID vaccine -we don’t currently distinguish between the third dose and the booster.

Boosters can give an increased measure of protection, around 95% according to studies by Pfizer. It is really just the third dose of the vaccine which most of us need to get maximum protection.

If you want to learn more about how vaccines are working in those with compromised immune systems read the summary on our website, where there’s also a link to watch again.

How to get your booster jab?

If you’re in England and eligible for the booster, NHS England are offering a walk-in service so you can get your booster without an appointment -you can find your nearest vaccination centre on the NHS website. Alternatively, you can book online

If you’re in Scotland or Wales, you’ll need to wait to be invited for your booster vaccine by your local healthcare provider. 

If you’re in the USA and eligible for the booster, you can find your nearest vaccine center on the CDC website.

Do I need a booster if I’ve already had COVID?

Although an infection and 2 vaccinations gives a high level of protection we advise everyone to get the booster after 6 months to maximise protection for yourself and those around you.


What’s the risk of getting COVID after a booster jab?

If you’ve previously had a COVID infection, or been vaccinated, we found that almost 99% of our contributors don’t get reinfected after receiving their booster. Breakthrough infections can still happen, but the rates are brought right down after subsequent vaccine doses. This data is based on Pfizer jabs only from our app population, using reports from September 2021 when boosters were first rolled out.

  1. 1 dose 10 % of people got infected 
  2. 2 doses 4% of people got infected
  3. 3 doses 1.5% of people got infected

Remember you don’t get the full benefit until 2 weeks after your jab. Like the first and second dose, no vaccine gives 100% protection, so it’s always crucial to avoid complacency and practice some cautionary measures in certain situations.

What are the after effects of a COVID booster vaccine?

We compared the booster jab side effects with after effects reported in those 50+ who received the 1st and 2nd dose of Pfizer, since all boosters currently being offered are Pfizer.  

We found that 14% of users reported a systemic (whole body) after effect. Compared with 1st (10.6%) and 2nd (16.4%) Pfizer dose. This shows that not so many people get after effects after receiving the booster, which is reassuring since there’s less chance of you getting really ill.

We also found that if you were previously infected, you were more likely to have a worse response to the booster jab. These findings reflect what we found with the 1st and 2nd dose. This was the case for people who had been infected at any time -either before or after their first vaccines. 

We’re currently investigating to see whether having a bigger reaction after your vaccine doses indicates higher protection due to a stronger immune response. We’ll bring you updates on our analysis as soon as we have them.

How important are booster jabs?

Having seen how the booster programme in Israel now seems to be working in bringing down overall cases, we can see that the third vaccine is our way out of high rates, together with modest restrictions in certain situations, and our data on safety is reassuring.

Thank you for continuing to log daily, your contributions are invaluable and crucial in helping us to continue our research.

Stay safe and keep logging!

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