A study of highly vaccinated healthcare workers found the more contagious Delta variant, combined the end of a masking mandate, was associated with increased breakthrough cases, paralleling exponential rise of COVID-19 infections in San Diego community, according to a news release from UC San Diego Health.
In a letter to the New England Journal of Medicine, an interdisciplinary team of physicians and public health experts at University of California San Diego measured the effectiveness of COVID-19 mRNA vaccines among healthcare workers at UC San Diego Health, most notably during the emergence of the highly transmissible delta virus variant and coincident with the end of the state’s mask mandate, allowing fully vaccinated persons to forgo face coverings in most places.
The letter’s authors report that the effectiveness of both the Pfizer and Moderna mRNA COVID-19 vaccines significantly waned over time. Both vaccines were granted emergency use authorization by the U.S. Food and Drug Administration (FDA) in December 2020.
In the letter, the authors note that from March through June 2021 vaccine effectiveness against symptomatic infection was estimated to exceed 90%; by July, however, it had fallen to approximately 65%.
In December 2020, workers at UC San Diego Health, like the population overall, began experiencing a surge of SARS-CoV-2 infections, the virus that causes COVID-19.
The situation improved significantly after UC San Diego Health began to inoculate employees using the Pfizer and Moderna vaccines. By March 2021, 76% of workers were fully vaccinated, rising to 83% by July 2021.
Concomitant with increased vaccination coverage was a decline between March and June in the number of workers reporting at least one symptom of COVID-19 and a positive PCR test. That number declined to fewer than 30 employees per month.
In July 2021, however, cases among this highly vaccinated population began to rise again, coincident with the emerging dominance of the delta variant in San Diego and the ending of California’s masking mandate on June 15. By July, 125 workers had been diagnosed with SARS-CoV-2 and unlike in previous months when approximately 20% of these cases involved vaccinated workers, the percentage had risen to 75%.
Notably, the vaccines still provide significant protection from severe infection outcomes, such as hospitalization and death. Among the UC San Diego Health employee cases documented, no hospitalizations were reported in vaccinated individuals and only one among unvaccinated persons.
Vaccine effectiveness was linked to the passage of time. For workers diagnosed in July, those who became fully vaccinated in January and February had higher infection rates than those vaccinated later in March through May. The infection rate among unvaccinated persons has remained consistently higher than for any vaccinated group, although the difference in rates between the two groups has decreased over time.