Some side effects of the Covid jab are almost indistinguishable from early symptoms of the virus itself, scientists have warned.
Researchers fear it may lead to people inadvertently spreading coronavirus, and say people should isolate if they get ill after getting the jab and get tested.
Vaccines do not offer immediate protection, with the body taking up to two weeks to learn how to fight off the virus. Even then, they may not stop people getting symptoms of the virus if they catch it.
Up to one in 10 people can suffer side effects after the jab including headache, fatigue, and a fever. King's College London experts say these side effects are also early warning signs of Covid.
Although UK guidance only officially recognises three symptoms of Covid, surveillance studies have suggested it causes dozens.
The academics found 1 per cent of the people who reported symptoms after the jab actually tested positive.
Professor Emma Duncan said: 'Vaccination remains extremely important, to help protect yourself, your family, and your community from Covid.
'However, if you do have symptoms afterwards, you should not assume that it’s just side effects of the vaccination although fortunately this was the most likely outcome.'
The warning is timely, with the UK currently embarking on a huge Covid booster programme. Cases of the virus in Britain are also on the rise, bolstering fears that the Omicron variant is already taking hold in the community.
This chart shows the symptoms reported by vaccinated people in the seven days after they received their jab. The orange bars indicate the symptoms of those who tested positive for Covid, while the teal shows the symptoms of those who tested negative. It shows that in most cases the symptoms between the groups were very similar and there is no way to tell if a person is Covid positive or suffering a vaccine side-effect without a test. A * indicates a symptom for which there was statistical difference
This heatmap graphic shows on which of the seven after a jab a person was most likely to report a symptom. The left shows the findings for those who tested positive and the right shows those who tested negative. They are both very similar
The research, part of the ZOE Covid Study, examined data from 362,770 people in the UK who were vaccinated between December 8 and May 17 and who reported at least one potential Covid symptom in the days after they were jabbed.
Of this group, 14,842 people took a Covid test with 150 subsequently testing positive for the virus.
The scientists say people who think they only suffering a side-affect of the vaccine may actually have Covid and could be unintentionally spreading it (stock image)
After examining the data the scientists, who published their findings in the journal eClinical Medicine, said there was no way to tell if the symptoms people reported were from Covid or a side effect of the jab, unless a test was taken.
Professor Duncan said: 'You should check to make sure you don’t have early Covid, by getting tested.
'This is particularly important now, when the UK is experiencing high levels of infection circulating in the community and particularly while we are still learning about the new variant, Omicron.'
She added that it was important to remember that vaccines take time to work, and urged people to be responsible to other people and get a test.
Professor Duncan said: 'It takes time to develop immunity after vaccination; and if you are infected it’s important to know this – both for your own sake, so that you can receive appropriate care, and for the sake of other people, so that you can ensure you don’t pass it along to vulnerable individuals.
'Being tested if you have symptoms – even if you’ve been vaccinated – will help stop the spread of Covid.'
The scientists did not say if their findings differed by the type of vaccine received.
People can get a free PCR Covid test if they are experiencing a high temperature, a new continuous cough, or loss or change in their sense of taste or smell, classic symptoms of a Covid infection.
The NHS says some of the most common side effects of getting a Covid jab are, a sore arm at the injection site, fatigue, aches and pains, and feeling or being sick.
People can also get a temperature in the days immediately following
