British scientists conducted a comparative analysis of the symptoms of early COVID-19 and the side effects of four vaccines used in the UK. The authors note that although coronavirus infection has characteristic manifestations, such as loss of smell, in general, post-vaccination symptoms are sometimes difficult to distinguish from the onset of the disease, especially when infection with the virus occurs between two doses of the vaccine. The research results are published on the medRxiv.org preprint server.


Due to the fact that about one in three people in the UK who received vaccines against COVID-19 had side effects after that, and about one percent of those vaccinated did get sick, it is important to clearly distinguish the symptoms of an early infection from the effect of vaccination, doctors say.


Researchers from King’s College London analyzed publications from clinical trials of vaccines from January 1, 2020 to June 21, 2021. The authors focused on four vaccines used in the UK: two mRNA vaccines, Pfizer and Moderna; and two adenoviral vector vaccines, AstraZeneca and Janssen.
In total, the sample included about 15 thousand people who received one of these vaccines. About a quarter of them – about 4,000 people – experienced what they described as “covid-like” after being vaccinated. Two percent of them tested positive for the PCR test. Of the remaining 11,000 who did not have covid-like symptoms, 0.8 percent tested positive. In other words, in ordinary life, 88 people out of 11 thousand vaccinated, would calmly walk the streets, not suspecting that they are infected.


Scientists attribute fatigue, headache and muscle pain, and fever to common symptoms that occur both after vaccination and with early COVID-19. These symptoms are noted in 76 percent of those vaccinated. For young people, this percentage is higher – up to 83 percent.
The difference is that on the third day after the vaccine, these symptoms usually disappear, and with COVID-19, on the contrary, they begin to increase and reach a maximum on the seventh day after infection. Infected people also get worse sore throat, sneezing and hoarseness over time.


The researchers note that these dates are approximate, since in no case was the exact time of infection known. In addition, people who were infected before vaccination did not know about this, which means that the disease was asymptomatic. All this complicates the development of unambiguous criteria by which one can distinguish between the symptoms of early covid and the side effects of vaccination.
“Post-vaccination symptoms cannot be clinically distinguished from early SARS-CoV-2 infection. Our study highlights the critical importance of testing people with symptoms, even if they have recently been vaccinated, for early detection of coronavirus infection and preventing future waves of COVID-19,” they write the authors of the article, who believe that only a PCR test can give an unambiguous answer in any case.

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