Vaccinated healthcare workers have a risk of being infected with SARS-CoV-2 of about 1%, a rate that is much higher than that reported in clinical studies.
Multiple reasons can explain this difference. However, emphasis should be placed on the paramount importance of protective measures, such as wearing a facemask, social distancing, daily symptom checking, and regular testing.
Papers published in the New England Journal of Medicine (NEJM) on the two messenger RNA vaccines—Pfizer’s and Moderna’s—showed an efficacy of 95% and 94.1%, respectively, in preventing symptomatic COVID-19 cases at 7 (Pfizer) and 14 (Moderna) days after the second dose.
Healthcare workers at two California Universities (San Diego and Los Angeles) began vaccination in December 2020 along with a very-low-threshold strategy that required testing after the slightest of symptoms and a systematic search for asymptomatic cases with a weekly PCR testing.
Between December 2020 and February 2021, about 21,184 workers received both doses of one of these messenger RNA vaccines.
Only 8 individuals had a positive PCR test result between 8 and 14 days after the second dose, and only 7 beyond 15 days.
In this cohort, the absolute risk of testing positive for SARS-CoV-2 was 1.19% in San Diego, and 0.97% in Los Angeles.
These risk rates are above those reported in both published papers on the efficacy of these vaccines. Multiple explanations may arise, but the two most obvious are extensive testing to diagnose asymptomatic patients and higher exposure to the virus compared with the general study population.
These data support vaccine efficacy, but also make clear the importance of maintaining all protective measures.
Original Title: SARS-CoV-2 Infection after Vaccination in Health Care Workers in California.
Reference: Jocelyn Keehner et al. N Engl J Med. 2021 May 6;384(18):1774-1775. doi: 10.1056/NEJMc2101927.