Resurgence in SARS-CoV-2 Infections in HCWs Tied to Many Factors

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Waning immunity to SARS-CoV-2 vaccination, coupled with the highly transmissible Delta variant and the end of mask mandates, likely contributed to a resurgence of SARS-CoV-2 infections among a highly vaccinated group of healthcare workers in California, according to a report released Wednesday.

“Our findings underline the importance of rapidly reinstating nonpharmaceutical interventions, such as indoor masking and intensive testing strategies, in addition to continued efforts to increase vaccinations, as strategies to prevent avoidable illness and deaths and to avoid mass disruptions to society during the spread of this formidable variant,” the investigators say.

“Furthermore, if our findings on waning immunity are verified in other settings, booster doses may be indicated,” say Francesca J. Torriani, MD, and colleagues at the University of California San Diego Health (UCSDH).

The study was published online September 1 in The New England Journal of Medicine.

Multiple Factors Implicated in Resurgence

The UCSDH workforce experienced a huge uptick in severe SARS-CoV-2 infections in December 2020. Immunization with mRNA vaccines started in mid-December 2020. By March, 76% of the workforce had been fully vaccinated; by July, that percentage had risen to 83%.

By early February of this year, the rate of SARS-CoV-2 infections had dropped sharply. Fewer than 30 healthcare workers tested positive for the virus each month between March and June.

However, coinciding with the end of California’s mask mandate on June 15 and the rapid takeover of the Delta variant, which made up over 95% of UCSDH isolates by the end of July, infections in the workforce increased rapidly, including cases among fully vaccinated individuals.

Between March 1 and July 31, 2021, 227 UCSDH healthcare workers tested positive for SARS-CoV-2 by reverse-transcriptase–quantitative polymerase-chain-reaction assay of nasal swabs.

More than half of them (130, 57.3%) were fully vaccinated.

Symptoms were present in 109 of the 130 fully vaccinated workers (83.8%) and in 80 of the 90 unvaccinated workers (88.9%). The remaining seven workers were only partially vaccinated.

None of the healthcare workers with SARS-CoV-2 infection died, but one unvaccinated individual was hospitalized for SARS-CoV-2-related symptoms.

The research team calculated vaccine effectiveness each month from March through July. They analyzed July case rates according to the month in which workers with COVID-19 completed the vaccination series.

For healthcare workers who completed vaccination in January or February, the attack rate (per 1000 persons) was 6.7 (95% CI: 5.9 – 7.8). By comparison, the attack rate was 3.7 (95% CI: 2.5 – 5.7) among workers who completed vaccination from March through May.

In contrast, among unvaccinated healthcare workers, the July attack rate was 16.4 per 1000 persons (95% CI: 11.8 – 22.9).

The investigators note that the SARS CoV-2 mRNA vaccines from Pfizer and Moderna were shown to have efficacy rates of 95% and 94.1%, respectively, in the initial clinical trials. For the Pfizer vaccine, efficacy is sustained, albeit efficacy slightly decreases (84%) 4 months after the second dose.

A recent study from England, where the dosing interval extends to 12 weeks, found that the rate of preserved vaccine effectiveness against symptomatic disease associated with the Delta variant was 88%.

“As observed by others in populations that received mRNA vaccines according to standard emergency use authorization intervals, our data suggest that vaccine effectiveness against any symptomatic disease is considerably lower against the delta variant and may wane over time since vaccination,” Torriani and colleagues note in their article.

The study had no specific funding. Disclosures of the authors’ relevant financial relationships are available on

N Engl J Med. Published online September 1, 2021. Full text

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