Covid-19: Delta infections threaten herd immunity vaccine strategy

Epidemiologists are adjusting their expectations for the future course of the pandemic after data from a recent outbreak in Massachusetts in the US suggest that while vaccination remains highly protective against the worst consequences of infection, it may not be sufficient on its own to stop the spread of the delta variant.

Testing conducted among Massachusetts residents during an outbreak in Provincetown, a popular weekend getaway spot, from 3 to 17 July found that 75% of those infected were fully vaccinated, in a state where 69% of adults were fully vaccinated.1

Among the vaccinated with breakthrough infections, the difficulty of detecting virus in the nasal passages, known as the cycle threshold value, was almost identical to that seen in the unvaccinated. This finding suggests that both groups carried equal viral loads and were equally likely to pass on their infections, the US Centers for Disease Control and Prevention (CDC) warned.

The results support claims that vaccinated people are playing a role in the summer surge in delta variant …