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Pfizer: Vaccine Effectiveness against Infection Drops to 47% after 5 Months

A Pfizer-funded study by S.Y. Tartof et al titled “Effectiveness of mRNA BNT162b2 COVID-19 vaccine up to 6 months in a large integrated health system in the USA: a retrospective cohort study” was published on October 4. The study period was from 14 December 2020 to 8 August 2021. It included over 3.4 million individuals.

Over the entire study period, fully vaccinated individuals had an adjusted vaccine effectiveness of 73% (95% CI 72–74) against SARS-CoV-2 infections and 90% (89–92) against COVID-19-related hospital admissions (appendix pp 6–7).

That is more-or-less expected but the effectiveness against infection “for the fully vaccinated decreased with increasing time since vaccination, declining from 88% (95% CI 86–89) during the first month after full vaccination to 47% (43–51) after 5 months”.

The effectiveness against hospital admissions remained at 87% to 88% during the study period.

As for the other variants, effectiveness against infection was high at 93% after 1 month of vaccination before dropping significantly.

At 4 months after full vaccination, vaccine effectiveness against delta infections declined to 53% (95% CI 39–65) and vaccine effectiveness against other variants declined to 67% (45–80; p=0·25).

So, the only positive note from this study is that it seems to be effective against hospitalizations for at least 5 months. However, that says nothing about long-term safety or transmissivity if one happens to be infected even if not hospitalized.

Figure 2A: Adjusted estimated vaccine effectiveness against SARS-CoV-2 infections.
Figure 2A: Adjusted estimated vaccine effectiveness against SARS-CoV-2 infections.

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