The findings, published last week in the eClinical Medicine journal, show there is no way to tell if the symptoms are from the disease or an adverse reaction to the shot unless a screening test is taken. Researchers at King’s College London have raised concerns that the recently vaccinated could be inadvertently spreading the virus, thinking their symptoms were from the jab.

According to the UK’s Medicines and Healthcare products Regulatory Agency (MHRA), more than one in 10 people can suffer minor adverse reactions to the vaccines – and the researchers found that 1% of the 362,770 people who reported symptoms in the week following the jab actually had Covid-19.

The study was based on data from those vaccinated with the Pfizer and AstraZeneca vaccines between December 2020 and May 2021.

The findings come amid a rise in cases and ahead of a high-profile booster vaccination drive for all adults in the UK, as officials worry about the potential impact of the virus’ Omicron variant.

Due to the higher risk of community transmission, researchers warn that it is more crucial than ever to check whether post-vaccination symptoms are in fact due to Covid-19. They noted that immunity to the virus does not occur immediately after getting the shot.

To identify the differences between early Covid infections and possible vaccine side effects, the scientists used machine learning models to review some 28 symptoms associated with the disease. They also analyzed three core clinical symptoms emphasized by the NHS: high temperature, new continuous cough, and loss/change in sense of smell.

“Both the machine learning models and the clinical model found it difficult to differentiate between symptoms associated with a positive test result and vaccine side effects. This is why anyone with symptoms should have a test. Ultimately, this will help us avoid spreading the virus this Christmas,” study co-author Dr. Liane Canas said.

The study did not reveal if the findings varied according to the particular vaccine received.