The delta variant is dangerous. A third vaccine dose could be helpful for those with weaker immune systems, but giving third doses to healthy people would be wildly counterproductive.

This week, Pfizer released early results showing that a third dose of its Covid-19 vaccine with BioNTech can dramatically increase antibody levels several months after vaccination, particularly among older people. A third Pfizer-BioNTech vaccine shot, in other words, could help some people fight off the delta variant, the pharmaceutical company told investors. Pfizer plans to file for an emergency use authorization for a third dose in “coming weeks,” a spokesperson told me.

The news followed the revelation from U.S. health officials on Tuesday that some fully vaccinated people who become sick with the delta variant may be just as infectious as unvaccinated people, based on the so-called viral load in their samples. The U.S. Centers for Disease Control and Prevention promptly reversed course on its masking guidelines to recommend vaccinated people still wear masks in some circumstances.

Figuring out whether another vaccine dose is a good idea, however, is far from simple. It’s normal for antibodies to begin fading after a few months, and that doesn’t mean the vaccines aren’t working well. It’s also not clear how the CDC counts viral loads among so-called breakthrough cases—i.e., vaccinated people who get sick with Covid. Because the CDC is tracking only breakthrough cases resulting in hospitalization or death, it’s likely they are talking about high viral loads among the seriously sick, not asymptomatic or mildly symptomatic cases. Cases of fully vaccinated people getting that sick are still rare, often occurring among a very small group of immune-suppressed people. They might indeed benefit from another dose. That doesn’t mean everyone else should line up for an additional dose, too. Read more at The New Republic.