Delta Hit So Hard Because Too Few Young Adults Were Vaccinated

The surge caused by the delta variant wasn’t only due to unvaccinated younger adults, but they played a substantial role

Photo: Mufid Majnun/Unsplash

When the delta variant reached the U.S. and caused another huge surge of infections, a key question arose: Is delta more virulent — does it cause more severe disease — than previous variants? Today, about six months after delta infections first appeared in the U.S., the evidence still doesn’t point to a definite answer to this question, but recent evidence suggests delta doesn’t cause any worse disease than alpha did. Rather, it’s that delta looked more severe at first because so many younger adults weren’t vaccinated when it arrived.

Evidence from early in the summer from Scotland found that unvaccinated people were roughly twice as likely to end up hospitalized from delta compared to alpha, the previously dominant variant in the U.S. Then studies from England and Canada two months later appeared to confirm those findings, which U.S. media picked up on. But it’s difficult to tease out all the reasons there might be higher numbers of hospitalizations or even a greater risk of hospitalization from a new variant. Disease severity of the variant is just one of those.