Slow global COVID-19 vaccination rates are raising concerns that worse variants of the coronavirus could be percolating, ready to rip into the world before herd immunity can diminish their impact.

Why it matters: The U.S. aims to at least partially vaccinate 70% of adults by July 4, a move expected to accelerate the current drop of new infections here. But, variants are the wild card, and in a global pandemic where only about 8% of all people have received one dose, the virus will continue mutating unabated.

How it works: Viruses face selective pressures thatcause them to mutate in order to spread better in the population and to escape human immunity, says Sarah Cobey, associate professor of ecology and evolution at the University of Chicago.

• "We're seeing both right now," she says.
• It's unclear if SARS-CoV-2 will evolve in the long term as the type of virus that branches out into multitudes of variants that coexist or if it will have more of a replacement pattern, Cobey adds.

Where it stands: The CDC currently says there are five variants of concern and eight variants of interest in the United States.

• Two variants of concern — New York and California — may be dropping off and "on their way to extinction," Topol says.
• Three variants raise more worries — those originally discovered in the U.K. (B.1.1.7), Brazil (P.1) and South Africa (B.1.351) — partly because "they accrued many mutations, over a dozen, almost instantaneously," says Josh Schiffer, an infectious disease expert at Fred Hutchinson Cancer Research Center.
• These three variants show varying levels of increased infectiousness, particularly B.1.1.7. Plus, P.1 and B.1.351 may be more able to evade the immune system or vaccination properties, Schiffer adds, although more data is needed.
• The CDC is closely watching several versions of B.1.617, a variant first detected in India that may be linked to the surge in cases there now.

"We were lucky because we vaccinated ahead of the onslaught [of the U.K. strain]. Otherwise we would have been in trouble. That's the superspreader strain," Topol says.

• Schiffer agrees partial herd immunity is causing the level of new infections in the U.S. to drop despite the highly infectious B.1.1.7's prevalence. "In the absence of vaccination, it's very likely that many places in the United States would look exactly like India right now with the new variants."
• "We're clearly seeing really pronounced signals of positive selection for increased transmissibility and what looks like some amount of immune escape," although this was not unexpected, Cobey adds.

What to watch: "Rapid vaccination is critically important. ... Even with partial protection you can achieve higher degrees of herd immunity," Schiffer says. "When I think of herd immunity, I don't think of it as an all-or-none phenomenon. I think of it as a dimmer switch."

• "The factories for generating new variants are areas that are getting hit very hard. If there is a new variant that's terrible — that ruins 2022 and brings us back to very dark times — it's almost a guarantee that it's percolating in an area of the world that's getting hit very hard now," Schiffer says.
• "The one thing that could happen, but hasn't happened yet, is to have a superspreader variant like B.1.1.7 with very powerful immune evasion. ... Will we see that? I don't know. Hopefully we'll never see that monster," Topol says.

Yes, but: The U.S. appears to be experiencing a drop in vaccination demand, despite the spread of variants.

• A new study in The New England Journal of Medicine shows the importance of people getting their second dose in fighting off the variants — but some Americans are not taking this step.
• And foreign nations are struggling to get access to vaccines, with the U.S. only now starting the process to fill in the vaccine diplomacy void.

The bottom line: "It's just next to impossible to predict what's going to happen next," Schiffer says.

• "I think the likelihood that we would have a variant that emerges that is worse than the ones we're dealing with now is much higher if you have a higher circulating number of infections," such as what's happening in Latin America, India and Asia.

Go deeper:

• Check out our live Coronavirus Variant Tracker
• The race between COVID vaccines and emerging variants
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