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Cases of infection even in vaccinated patients

The Pfizer-BioNTech vaccine has high efficacy against symptomatic COVID-19 infections. However, there have been cases of infection by SARS-CoV-2 in fully vaccinated individuals (2 doses), notably amongst health care staff who in general have greater exposure to the virus. Israeli researchers have looked at these types of cases in order to describe and analyse them, and thereby better understand them.

The researchers worked in the largest medical centre in the country (the Sheba Medical Centre, Ramat Gan, Israel) which employs more than 12 000 people. They carried out qPCR diagnostic tests, serological tests (evaluation of the presence of anti-SARS-CoV-2 antibodies) and virus genome sequencing (to determine the variant).

Between December 2020 and April 2021, 91% of the staff were vaccinated with 2 doses of the Pfizer-BioNTech vaccine. Out of 1 497 qPCR tests, 38 were positive (2,6% of the total tests). In individuals who were vaccinated but infected, levels of anti-SARS-CoV-2 neutralising antibodies were lower than in non-infected vaccinated people. The majority of these cases were asymptomatic or with mild symptoms only, even though these symptoms persisted for several weeks in 19% of the individuals studied. 85% of cases involved the Alpha (UK) variant.   

In conclusion, the majority of the cases detected would not have been found if not for the close monitoring provided by this study, demonstrating that the vaccine gives effective protection against serious forms of COVID-19. The occurrence of infection correlates with lower levels of anti-SARS-CoV-2 antibodies, which may therefore serve as markers to help prevent infection in susceptible individuals.

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