Three-quarters of COVID cases were among the vaccinated in an outbreak during large public gatherings at Cape Cod island in Massachusetts, USA, with the Delta variant predominating, researchers found. This helped reverse the official US recommendations on mask-wearing when fully vaccinated.
Of 469 cases linked to numerous summer events and large summer gatherings in a small town, 346 (74%) happened in fully vaccinated people, and almost 80% of those cases were symptomatic, reported Catherine Brown, DVM, of the Massachusetts Department of Public Health, and colleagues.
There were five hospitalisations, four among fully vaccinated people, and no deaths. Of 133 cases with sequence information available, 89% were from the Delta variant, the authors wrote in the Morbidity and Mortality Weekly Report.
Moreover, vaccination coverage in Massachusetts was reported to be 69% as of July 3, they noted.
The data on this outbreak, along with the Delta variant, reportedly motivated the Centers for Disease Contol’s change in indoor masking guidance earlier last week.
CDC Director Rochelle Walensky, MD, noted that “rapid receipt and review of unpublished data” contributed to the guideline change.
The authors found that RT-PCR cycle threshold (Ct) values in vaccinated and unvaccinated people were comparable. Pointing to these high viral loads among the vaccinated in this case, Dr Walensky said it suggested that vaccinated individuals infected with Delta could still transmit the virus.
“This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation,” she said, saying it was “updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones.”
The authors detailed increased reports of COVID cases in Barnstable County, beginning on July 10, including those who were fully vaccinated. Many COVID patients reported attending large indoor and outdoor gatherings.
They identified a cluster of cases, 346 of them fully vaccinated, in Massachusetts residents, who tested COVID positive 14 days or sooner after travel to or staying in Barnstable County.
Cases were also reported from residents of other states who traveled to this town during the period, and reports of secondary transmission.
The authors urged stricter prevention measures at such events, given, “the potential risk of infection during attendance at large public gatherings that include travelers from many areas with differing levels of transmission.”
Source: MedPage Today
Journal information: Brown CM, et al “Outbreak of SARS-CoV-2 Infections, Including COVID-19 Vaccine Breakthrough Infections, Associated with Large Public Gatherings – Barnstable County, Massachusetts, July 2021” MMWR 2021; Published July 30, 2021.
