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Fully Vaccinated With Breakthrough Infection Can Transmit COVID-19

Peak viral load similar for fully vaccinated and unvaccinated index cases; viral load declines at faster rate among fully vaccinated individuals

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MONDAY, Nov. 1, 2021 (HealthDay News) -- Fully vaccinated individuals with breakthrough COVID-19 infection have a viral load similar to unvaccinated cases and can transmit infection in a household setting, according to a study published online Oct. 28 in The Lancet Infectious Diseases.

Anika Singanayagam, Ph.D., from the National Heart and Lung Institute in London, and colleagues examined transmission and viral load kinetics in vaccinated and unvaccinated individuals with mild delta variant infection. Transmission risk was assessed for 231 contacts exposed to 162 delta variant-infected index cases. In addition, viral load trajectories were compared from 29 fully vaccinated people with delta variant infection and unvaccinated individuals with delta, alpha, and pre-alpha variant infections (16, 39, and 49, respectively).

The researchers found that the secondary attack rate (SAR) in household contacts exposed to the delta variant was 25 and 38 percent in fully vaccinated and unvaccinated individuals, respectively. The SAR among household contacts was similar for those exposed to fully vaccinated versus unvaccinated index cases (25 versus 23 percent). Of the 31 infections in fully vaccinated household contacts, 39 percent arose from fully vaccinated epidemiologically linked index cases. Peak viral load did not differ by vaccination status or variant type; a modest increase was seen with age. Compared with unvaccinated individuals with pre-alpha, alpha, or delta variant infections, fully vaccinated individuals with delta variant infection had a faster mean rate of viral load decline.

"Our findings suggest that vaccination alone is not sufficient to prevent all transmission of the delta variant in the household setting, where exposure is close and prolonged," the authors write.

One author disclosed financial ties to the pharmaceutical industry.

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