A new study by Harvard University has found that a coronavirus variant that spreads quickly can be worse than one that evades coronavirus vaccines.

The study — published in the medical journal Cell — found that variants that can escape acquired immunity and vaccines can cause a less severe pandemic because they would likely cause breakthrough cases or reinfections, which tend to have mild symptoms.

  • Meanwhile, variants that spread quickly can move through susceptible populations, leading to more infections and deaths from the virus.
  • “Partial immune escape can have severe consequences, but mainly when paired with enhanced transmissibility,” the study said. “Our findings suggest that variants with enhanced transmissibility have a strong tendency to invade and can significantly worsen an epidemic.”

To find this, the researchers used a mathematical model to see how COVID-19 variants would impact populations.

Mary Bushman, co-author of the paper, told Newsweek she was surprised by the results. But, she said, transmissibility is critical because it allows the variant to “catch up” to existing strains, which would then allow it to “do a lot of damage.”

The worst variant, she said, is one that evades vaccines and can spread quickly.

  • “Thus far, evidence of immune escape — the ability of a variant to evade the immune system and cause reinfections or breakthrough infections — has been a red flag,” Bushman said, according to an interview with the Harvard T.H. Chan School of Public Health. “Our findings say it’s maybe more of a yellow flag — this is not such a big deal on its own. But when it’s combined with enhanced transmissibility, then it can be a really big deal.”

Researchers are still monitoring any potential coronavirus variants that pop up in the near future. Dr. Mark Dybul, a professor at Georgetown University Medical Center’s Department of Medicine and immunologist, told Fortune that there will likely be a COVID-19 vaccine-resistant variant by spring 2022.

  • “The faster we get boosted, the better off we’ll be for the next couple of months,” he said. “Sadly, every prediction I’ve made has pretty much come true. I hope I’m wrong this time, but I think by March, April, May, we will have a fully vaccine-resistant variant. There’s simply no way you can have such low rates of vaccination around the world with the virus ping-ponging between vaccinated and unvaccinated people. I’m an immunologist. The probability of us seeing a vaccine-resistant strain is very high.”
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Dr. Anthony Fauci has also worried about a “monster variant” that could arrive, which would make the delta variant — seen as one of the most highly-transmissible variants out there — look weak.

  • “There’s always a risk of, as you get more circulation of the virus in the community, that you’ll get enough accumulation of new mutations to get a variant substantially different than the ones we’re seeing now,” Fauci said on the MSNBC’s “Morning Joe.”