Most hospitalized vaccinated patients likely acquired COVID-19 infection before development of immunity |
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Most hospitalized vaccinated patients likely acquired COVID-19 infection before development of immunity

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Most people who had taken the vaccine and were hospitalized for COVID-19 likely had developed the infection just before or about the time of the vaccination, as per a BMJ report.

Hospitalized vaccinated patients during the second wave of the UK were evaluated by the International Severe Acute Respiratory Infection Consortium Clinical Characterisation Protocol (ISARIC4C) using data available up to April 10, 2021. Of the 99,445 hospitalized patients enrolled in the study, 3842 (7.3%) had received the vaccine. Time to onset of symptoms and also mortality were analyzed for the vaccinated patients.

  • 729 (40%) symptomatic patients developed COVID-19 symptoms 0-7 days after the vaccination
  • 352 (19%) developed symptoms 8-14 days post-vaccination
  • 211 (12%) patients developed symptoms 15-21 days after vaccination
  • 526 (29%) developed symptoms more than 21 days after vaccination

Since the median incubation period for SARS-CoV-2 is approximately 5 days, these observations suggest that the majority of patients had been infected before they developed full immunity as this is not enough time (0-7 days) for the immunity to develop. And those who developed symptoms 8-14 days after the vaccine were infected before the immunity had fully developed. These infections are unlikely to be vaccine failures.

However, there is a possibility that those who developed symptoms 15-21 days or >21 days post-vaccine were cases of vaccine failure. But, this is along expected lines as vaccines were not 100% effective.

ISARIC4C report states that the “elderly and vulnerable people who had been shielding, may have inadvertently been exposed and infected either through the end-to-end process of vaccination, or shortly after vaccination through behavioural changes where they wrongly assume they are immune.”

Among those who developed symptoms ≥21 days after the vaccination, 28% (113/400) died with COVID-19. Of these, 82 were in the “frail elderly” group. The report said, “Mortality appears to remain high for people in high risk vaccination tiers who are admitted to hospital with SARS-CoV-2 infection despite vaccination 21 day or more previously.”

These findings reiterate the importance of maintaining social distancing, even after vaccination, to minimize the risk of infection as most infections had occurred within 14 days of vaccination before immunity fully develops.

Deborah Dunn-Walters, chair of the COVID-19 taskforce at the British Society for Immunology and professor of immunology at the University of Surrey, said, “A very small number of people were hospitalized 21 days post-vaccination, and it’s these people that we need to examine in more detail to understand why the vaccine did not afford them full protection.”

(Source: BMJ 2021;373:n1127, published 30 April 2021)


Dr KK Aggarwal

President CMAAO, HCFI and Past National President IMA

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