Reasons Behind Increased COVID-19 Infections in a Highly Vaccinated Population

In December 2020, the healthcare workforce at San Diego University saw an unexpected increase in COVID-19 infections. Concurrently, this same month, one of the most ambitious vaccination campaigns ever was taking off, one which managed to inoculate 76% of the population by March 2021 and 87% by June same year. 

Las razones detrás del aumento de los diagnósticos de COVID-19 en una población altamente vacunada

Infections saw a significant reduction as of February and plateaued until June. 

However, the rapid spread of the Delta variant (B.1.617.2) together with more flexible safety policies, such as the end of California’s mask mandate, resulted in the isolation of 95% of San Diego’s healthcare workers by July, and a fast increase in COVID cases even among fully vaccinated workers. 

In this context, it was necessary to gather more data to better prepare and protect against this disease. 

All healthcare workers with at least one symptom were PCR tested. Symptoms were present in 109 of the 130 workers that turned out positive and were fully vaccinated (83.8%). 90 unvaccinated workers resulted positive that same month, and 80 had symptoms (88.9%). 

There were no fatalities and only one required hospitalization (unvaccinated).

Vaccines Pfizer–BioNTech (BNT162b2) and Moderna (mRNA-1273) had previously shown 95% and 94.1% efficacy respectively, in clinical trials, but these efficacy rates dropped to 84% 4 months after the second shot.  Efficacy may wane over time since vaccination, possibly one of the reasons behind this resurgence.

Read also: New Evidence in Israel in Favor of Applying a Third Dose against COVID-19.

Another study in the UK showed over 90% efficacy for both mRNA vaccines, on the same lines of the original studies. 

Between March and June, we saw a rise in delta infections in the UK and together with it, a drop in vaccine efficacy to 65.5%.

There is no doubt that efficacy against both symptomatic and asymptomatic disease is considerably lower when it comes to the delta variant. Together with prior data, we could also assume efficacy will drop even lower over time. 

At present, we have more resources to combat the pandemic, but we should prioritize non-pharmaceutical prevention measures such as the mandatory use of face masks, intensive testing, continued efforts to persuade those who remain unvaccinated, avoiding massive events, etc. 

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Original Title: Resurgence of SARS-CoV-2 Infection in a Highly Vaccinated Health System Workforce.

Reference: Jocelyn Keehner et al. N Engl J Med. 2021 Sep 1;NEJMc2112981. Online ahead of print. doi: 10.1056/NEJMc2112981. 

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