Dr. Munster’s team showed that small aerosols traveled much longer distances than larger droplets and the Alpha variant was much more likely to cause new infections via aerosol transmission. The second study found that people infected with Alpha exhaled about 43 times more virus into tiny aerosols than those infected with older variants.

The studies compared the Alpha variant with the original virus or other older variants. But the results may also explain why the Delta variant is so contagious — and why it displaced all other versions of the virus.

“It really indicates that the virus is evolving to become more efficient at transmitting through the air,” said Linsey Marr, an expert in airborne viruses at Virginia Tech who was not involved in either study. “I wouldn’t be surprised if, with Delta, that factor were even higher.”

The ultratransmissibility of the variants may come down to a mix of factors. It may be that lower doses of the variants are required for infection, or that the variants replicate faster, or that more of the variant virus is exhaled into aerosols — or all three.