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We’re used to talking in absolute terms about how dangerous or contagious diseases are. After all, so many of the viruses and bacteria that seriously threaten human life have been around for hundreds or thousands of years, which means we have plenty of research. COVID-19 has been a different story.

Estimating how much of a hazard SARS-CoV-2 poses has been an ongoing challenge, especially as the virus has evolved new strains. Early estimates that COVID-19’s reproductive number (R0)—the number of people each sick person will infect, on average—was around 1.5-3.5 have largely held true. That made it more infectious than the flu or Ebola, but less so than SARS. The delta variant, however, is more than twice as infectious. Each sick person will infect an average of seven others, making it nearly as contagious as chickenpox.

graphic showing how contagious covid-19 is compared to other diseases
Updated August 2021. Graphic by Sara Chodosh

This makes the delta strain much more challenging to control than the original virus, even with a significant fraction of the population fully vaccinated. In part that’s because unvaccinated individuals tend to be clustered geographically, making it easier for the coronavirus to spread than if the susceptible people were evenly distributed across the globe. But it’s also because even vaccinated people can spread the virus. It’s unusual and much less likely to happen, but it’s not rare enough for our current vaccination rates to nip transmission in the bud.

We may now need to vaccinated upwards of 90 percent of the population to reach herd immunity. Earlier estimates were closer to 70 percent, but we have yet to even reach that benchmark: Just slightly over half of the US population is fully vaccinated.

This means that getting people vaccinated is more crucial than ever. The more time we give the virus to spread, the more likely it is that increasingly infectious—and potentially more deadly—strains can develop. Every single person who gets vaccinated makes it just a little bit harder for the delta variant to spread, which protects the most vulnerable among us. Wearing masks and minimizing social contact will also continue to help prevent the spread, especially in circles where people are unvaccinated. Remember: every precaution, no matter how small, makes a difference.

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