Vaccine after effects more common in those who already had COVID

February 4, 2021

According to the latest data from the ZOE COVID Symptom Study people who have had a previous COVID-19 infection are almost twice as likely to experience one or more mild whole body (systemic) after effects compared to people who didn’t have COVID-19 (33% vs 19%) from a Pfizer/BioNTech vaccine dose. The most common mild whole body (systemic) after effects experienced were fatigue (9%), headache (8%) and chills or shivers (4%). Most mild whole body (systemic) after effects appeared in the first two days after the vaccination and only 3% of people have any after effects beyond three days.

COVID-19 vaccines work by using a harmless version or component of the SARS-CoV-2 coronavirus to train the immune system, so when the virus is encountered the body is able to fight it off. This ‘training’ response can feel like some of the symptoms when the body is fighting off a real infection, including headaches, fever, chills or shivers, tiredness (fatigue), muscle or joint pains, diarrhoea and feeling sick (nausea). A stronger response may indicate evidence of an increased immune response.

Summary of findings:

  • A third (33%) of those who had previously had COVID-19 reported at least one whole body (systemic) after effect after 7 days compared to 19% of who had not previously had COVID-19
  • 37% of people experienced some local after effects such as pain or swelling near the site of the injection after first dose and 45% after the second dose
  • 14% of people reported at least one whole body (systemic) after effect within the 7 days from the first dose compared to 22% from the second dose, possibly indicating a stronger immune response after the second dose.
  • 13% of vaccinated men and 19% of vaccinated women reported at least one whole body (systemic) after effect within seven days
  • Under 55s more likely to experience whole body after effects than those over 55s (21% vs 14%)
  • 280,000 people have already logged their vaccine experience on the app

The data comes from a sub-sample of almost 40,000 vaccine doses, all first doses were administered before 4 January 2021 and logged in the ZOE COVID Symptom Study app. This is a subset of over 280,000 people, one of the largest vaccine datasets outside the trial settings where participants are carefully selected. Contributors logged any local after effects (in the arm) for seven days and logged systemic (whole body) after effects continuously after receiving the vaccine. This current data is for the Pfizer/BioNTech vaccine and most of those analysed were healthcare workers. 

The ZOE app also asked over one million contributors whether or not they want to receive a vaccine and if not, why. Whilst almost 95% said ‘yes’, just over 5% of respondents were either reported being unsure or unwilling to accept a vaccine. The main three reasons given by hesitant contributors were; long term side effects (50.2%), lack of knowledge (37.2%), and concerns about potential adverse reactions (30.9%).


Tim Spector OBE, lead scientist on the ZOE COVID Symptom Study app and Professor of Genetic Epidemiology at King’s College London, comments on vaccine data: 
“This data set is a unique look at those who have been vaccinated in the real world outside trials, and so far the post vaccine effects we see are mild and in the minority of people. It’s interesting to see that those with previous COVID are more likely to experience these mild after effects than naive subjects. This could be good news, as a larger response like this suggests that those getting a first dose after having had COVID are generating a stronger immune reaction and may get greater protection from just a single shot of the vaccine. We are urging as many people as possible to download the ZOE app and log their vaccines with us, so that we can independently monitor how we all react differently and how the vaccines impact the overall pandemic.” 


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