It was a long, dark winter of COVID-19 concerns, stoked by high post-holiday case counts and the American death tally exceeding 537,000 lives lost. But with spring arrived three vaccines — one each by ...

It's a question that scientists have been trying to answer since the start of the pandemic, one that is central to the rancorous political debates over coronavirus vaccine policies: How much immunity ...

Herd immunity occurs when enough people in a group are immune to a specific infection that it cannot spread easily from person to person. Vaccination programs have been successful in creating herd ...