Delta variant may have led to reduced vaccine efficacy among US health workers, new study finds

Researchers at University of California San Diego Health find that vaccine efficacy in US health workers dipped from 90% to 65.5%, recommend continued use of masks & intensive testing.

A healthcare worker receives the Pfizer-BioNTech Covid-19 vaccine in Ann Arbor, Michigan, US | Representational image | Bloomberg

New Delhi: The Delta variant may be one of the reasons behind a reduced vaccine efficacy among US healthcare workers — from 90 per cent in March to 65.5 per cent in July — say US scientists. They also recommended that use of face masks and intensive testing strategies continue even after two doses of vaccines, to prevent avoidable infections and deaths.

In an article published in The New England Journal of Medicine Wednesday, researchers from the University of California San Diego Health (UCSDH) pointed out that despite 83 per cent of their (UCSDH) healthcare workers being vaccinated, a higher than expected number of people got infected with Covid in July.

Vaccination in the US with mRNA vaccines began in mid-December 2020.

According to the researchers, infections had decreased dramatically by early February 2021, with less than 30 healthcare workers testing positive each month between March and June.

However, the state of California ended its mask mandate on 15 June, allowing people who have been vaccinated to work without facemasks. This coincided with the dominance of the Delta (B.1.617.2) variant, which had wreaked havoc in India during the second Covid wave.

The Delta variant accounted for over 95 per cent of UCSDH cases by the end of July, the researchers said.

Between 1 March and 31 July 2021, a total of 227 healthcare workers in the hospital tested positive for SARS-CoV-2. Of these, 130 (57.3 per cent), had been fully vaccinated, or received both vaccine doses, prior to being infected. Seven of the infected workers had been partially vaccinated.

As many as 109 of the 130 fully vaccinated workers and 80 of 90 unvaccinated workers displayed Covid symptoms. The remaining were asymptomatic.

No death was reported among the 227 health workers, and only one unvaccinated person had to be hospitalised for the infection.

“Our data suggests that vaccine effectiveness against any symptomatic disease is considerably lower against the Delta variant and may wane over time since vaccination,” the researchers said.

‘From over 90% vaccine efficacy fell to 65.5%’

Vaccine effectiveness was calculated for each month from March through July. The team found that vaccine effectiveness (in preventing infections) exceeded 90 per cent between March and June, but fell to 65.5 per cent in July.

The SARS CoV-2 mRNA vaccines — BNT162b2 (Pfizer–BioNTech) and mRNA-1273 (Moderna) — had shown efficacy rates of 95 per cent and 94.1 per cent, respectively, in their initial clinical trials late last year.

However, the change in vaccine effectiveness may not be only due to the emergence of the Delta variant. Researchers argue that it is likely that waning immunity over time, along with the end of masking requirements in California, could have brought about this reduced efficacy.

Moreover, the data in this study is from healthcare workers, who are at a greater risk of exposure to the virus than the general population.

“The 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 researchers wrote in the study.

They added that if other research groups are able to verify that the immunity induced by vaccines or previous exposure to the virus wanes over time, then booster doses of the vaccine may be needed.

(Edited by Poulomi Banerjee)