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(INDIANAPOLIS) – Half of all eligible Hoosiers are now vaccinated against COVID-19. But a more meaningful milestone is still a long way away.

The goal of vaccination isn’t just to keep you from getting sick, but to deny the virus the opportunity to mutate into a version that can beat the vaccine. That’s what scientists mean by “herd immunity.” Mutating and spreading is the virus’s R&D process. The more people are immune to the virus, the more it will hit dead ends, instead of finding incubators for new variants that may be able to pick the lock of the vaccine.

Community Health Network chief medical officer Ram Yeleti says the Delta variant, which is now responsible for three-quarters of new cases, illustrates the problem. Most of the variants have originated overseas, often in countries with low vaccination rates and more opportunities for the virus to spread. Delta was first detected in India, where just 6% of the population is fully vaccinated. The vaccines do protect against Delta, but the variant is far more contagious than past strains.

Yeleti says we don’t know yet exactly what it’ll take to achieve herd immunity, because it varies from one virus to the next. Based on past experience, though, it’s not 50%, but in the 70-to-80% range.

Just 24 counties have vaccinated more than half their eligible residents — no county has reached 70%. Hamilton County is closest, about 600 people away. And a million Indiana kids are too young to be vaccinated, meaning another two-million of those eligible would need the shot in order to reach 70%.

750,000 Hoosiers have already had and survived the virus, and Yeleti says that contributes to the quest for herd immunity, though the Centers for Disease Control still recommend those people get the shot. Immunity from having already had the virus isn’t believed to be as long-lasting as from the vaccine.

Just three states — Massachusetts, Maine and Vermont — have vaccinated 70% of eligible residents. No state has reached 70% of the total population, with Vermont the closest at 67%.