Decline in vaccine effectiveness tied to repealed mask mandates, delta variant

COVID vaccination rates right now are stagnant. At the same time, the Delta variant is highly transmissible and the dominant COVID strain in the U.S. It now accounts for 83 percent of all cases in the country.
Posted at 3:26 PM, Sep 01, 2021
and last updated 2021-09-01 18:26:58-04

SAN DIEGO (CNS) - A team of researchers at UC San Diego Wednesday published a letter stating that the effectiveness of both the Pfizer and Moderna COVID-19 vaccines have significantly waned over time, partially due to the ending of mask mandates and the highly contagious Delta variant.

The letter, which appeared in The New England Journal of Medicine, was written by an interdisciplinary team of physicians and public health experts at UCSD. They measured the effectiveness of COVID-19 mRNA vaccines among health workers at UCSD 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 people to forgo face coverings in most places.

The authors noted that from March through June, vaccine effectiveness against symptomatic infection was estimated to exceed 90%. By July, however, it had fallen to around 65%.

Both Pfizer and Moderna vaccines were granted emergency use authorization by the U.S. Food and Drug Administration last December, with vaccinations of the UCSD Health workforce beginning the same month for employees with direct, patient-facing duties.

"The decline in effectiveness is not entirely surprising," said co- senior author Dr. Francesca Torriani, a professor of clinical medicine in the Division of Infectious Diseases and Global Public Health in the UCSD School of Medicine and program director of Infection Prevention and Clinical Epidemiology at UCSD Health.

"Clinical trial data suggested decreased effectiveness would occur several months after full vaccination, but our findings indicate that confronted by the Delta variant, vaccine effectiveness for mildly symptomatic disease was considerably lower and waned six to eight months after completing vaccination," Torriani said.

In December, workers at UCSD 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 the hospital system began to inoculate employees using the two-dose vaccines. By March, 76% of UCSD Health workers were fully vaccinated, rising to 83% by July.

Along 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 test. That number declined to fewer than 30 employees per month.

In July, however, cases among the 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 the end of July, 125 workers had been diagnosed with SARS-CoV-2 and unlike in previous months when about 20% of cases involved vaccinated workers, the percentage had risen to 75%, the researchers said.

Notably, the vaccines still provide significant protection from severe infection outcomes, such as hospitalization and death. Among the UCSD Health employee cases documented, no hospitalizations were reported in vaccinated individuals and only one among unvaccinated persons.

"Unlike what was experienced with other variants, with the Delta variant parents are frequently getting infected by their young children, ages 5 to 11," said co-first author Dr. Lucy Horton, an assistant professor of medicine in the Division of Infectious Diseases and director of the UCSD Health COVID-19 case investigation and contact tracing team.

"Unvaccinated people are seven times more likely to test positive for COVID-19 than those who are fully vaccinated," Horton continued. "More importantly, while children rarely need medical attention, unvaccinated adults are 32 times more likely to require hospitalization compared to those who are fully vaccinated."

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 people has remained consistently higher than for any vaccinated group, although the difference in rates between the two groups has decreased over time.

"The dramatic change in vaccine effectiveness from June to July is likely due to a combination of factors," said co-author Dr. Nancy Binkin, a professor of epidemiology in the school of medicine and Herbert Wertheim School of Public Health and Human Longevity Science. "It's the emergence of the Delta variant and waning immunity over time, compounded by the end of broad masking requirements and the resulting greater exposure risk throughout the community."

Co-senior author Dr. Shira Abeles, an assistant professor of medicine in the Division of Infectious Diseases who has led the COVID-19 vaccination effort at UCSD Health, said the findings underscore the importance of rapidly reinstating key interventions, such as indoor masking and intensive testing strategies, plus continuing efforts to boost vaccination rates.

"Similar findings are being reported in other settings in the U.S. and internationally, and it is likely that booster doses will be necessary," Abeles said.