Updated: August 29, 2021 9:59:38 pm
Covid-19 vaccines remain effective, but their potency has diminished in recent months, according to a study at eight sites across the US. Scientists calculated vaccine effectiveness to be 80% in a large group of fully vaccinated frontline workers between December 2020 and August 2021, compared to 91% in earlier surveys, the University of Utah Health said in a press release. The estimates were based on RT-PCR testing and did not measure whether there were changes in efficacy in protecting against severe disease, including hospitalisation and death, it said.
The authors say one reason for the change could be waning immunity, a decrease in the strength of the body’s vaccine-activated defences against the virus. The difference may also reflect the fact that the vaccines are not as effective against the highly contagious Delta variant which, since June 2021, has become the most common cause of Covid-19 in the US.
The study was published on August 24 in the Morbidity Mortality Weekly Report from the US Centers for Disease Control and Prevention (CDC).
The HEROES-NETWORK sites were in Salt Lake City (Utah) Phoenix, Tucson and other areas in Arizona; Miami (Florida); Portland (Oregon); Duluth (Minnesota); and Temple (Texas). The network followed 4,136 healthcare personnel, first responders, and essential workers who had not previously had Covid-19. Study participants submitted samples for RT-PCR testing on a weekly basis and 2,976 participants were fully vaccinated within the study period, receiving the Pfizer-BioNtech (65%), Moderna (33%), or Johnson & Johnson (2%) vaccines. Test results from these groups between December 14, 2020, to August 14, 2021, show that:
Among unvaccinated study participants, 194 infections occurred in 181,357 person-days (combined total of number of days of testing for this group).
Among fully vaccinated participants, 34 infections occurred in 454,832 person-days.
During that time period, the vaccines were 80% effective for all fully vaccinated study participants, but preliminary data indicate that vaccines may wane in intensity over time with lower effectiveness after five or more months following full vaccination, the press release said.
Source: University of Utah Health
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