Why BA.5 became the dominant COVID variant

BA.5 is the most transmissible and immune-evasive subvariant yet. Here's how it took over the world

By Nicole Karlis

Senior Writer

Published August 10, 2022 6:39PM (EDT)

Surgical face mask with "BA.5" written on it, referring to the BA.5 variant of COVID-19 (CFOTO/Future Publishing via Getty Images)
Surgical face mask with "BA.5" written on it, referring to the BA.5 variant of COVID-19 (CFOTO/Future Publishing via Getty Images)

Never say the pandemic hasn't been educational, as COVID-19 has certainly given all of us a crash course in evolution. Charles Darwin's notion of the survival of the fittest, one of the tenets of his theory of evolution, epitomizes the manner in which subsequent mutations of the virus that are better-suited to spread have outcompeted each other — typically eradicating earlier, less-fit virus variants in the process. Indeed, from the primordial alpha variant to the delta variant to omicron, new variants emerge when unique mutations make a strain either more contagious, or more capable of evading immunity from previous infections and vaccines.

RNA viruses like SARS-CoV-2 are always mutating; every replication in a hosts' cells creates a moment for a chance mutation to emerge. While viruses are technically not alive, it is their nature to mutate and evolve as they infect hosts' cells and replicate. Indeed, this is how they survive.

In most cases, mutations are harmful to the virus' ability to reproduce, and thus often eliminated in the process of natural selection. However, if a mutation has a competitive advantage — like increased transmissibility — that mutation can outcompete a previous variant. Indeed, that is what has been driving the mutations we've seen evolve over time.

In early July, federal officials announced that BA.5 became the most dominant strain in the U.S., meaning that it makes up a majority of the infections. This came nearly four months after BA.2 was named the most dominant strain. Both are subvariants of the omicron variant.

According to data from the U.S. Centers for Disease Control and Prevention (CDC), BA.5 is now responsible for nearly 87 percent of COVID-19 infections in the U.S.

"The omicron sub-variant BA.5 is the worst version of the virus that we've seen."

But the latest variant to take hold across the world is — according to some infectious disease experts — the most concerning one yet.

"The omicron sub-variant BA.5 is the worst version of the virus that we've seen," Dr. Eric Topol, founder and director of the Scripps Research Translational Institute, wrote in late June. "It takes immune escape, already extensive, to the next level, and, as a function of that, enhanced transmissibility, well beyond Omicron (BA.1) and other Omicron family variants that we've seen (including BA.1.1, BA.2, BA.2.12.1, and BA.4)."

But just how much more transmissible is BA.5 than previous subvariants — some of which were also dubbed the most transmissible and/or concerning variants at the time?

One method for determining transmissibility involves calculating the average number of additional people infected by an already-infected person. For the original strain of COVID-19, researchers estimated that for each person who got infected with COVID-19, it would spread to about two to three more people. This number is what scientists call the R-naught [R0] value of a virus.

For the delta variant, which became the most dominant variant in late 2021, it had an R-naught of about 7. The omicron variants are even more transmissible than delta. The first version of omicron was estimated to have a R-naught value of 9.5, meaning one infected person would, in an uncontrolled situation, spread it to an average of 9.5 others. 

Scientists are still debating exactly how much more contagious BA.5 is than its predecessors. One Australian researcher believes that each person who got BA.5 could spread it to 18 others, but other researchers have disagreed with that claim.

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In Topol's article, he explains that BA.5 has become the dominant strain not merely because of its high transmissibility, but because of its ability to evade immunity, too, .

"You can see that the antigenic distance from BA.1 to BA.2 is far greater than the ancestral strain to Delta or Beta or Gamma," Topol explained, pointing to an antigenic map of the subvariant. Antigenic distance refers to how different a variants' genetics are compared to its peers; because of the way the immune system works, a strain that was genetically distant (compared to a strain that someone previously contracted or was vaccinated against) would be better at evading immunity. 

"This is the basis for the immune escape of BA.5 — our relatively poor recognition of and response to the spike protein," Topol continued. Spike proteins comprise little points that stick out of the side of the SARS-CoV-2 virus in artistic depictions, and such proteins are crucial to the virus' ability to enter human cells. Hence, BA.5's mutation in its viral spike protein has strengthened its ability to latch onto host cells and evade some immune responses.

Unfortunately, researchers are confident that re-infections with BA.5 are likely even if you've been infected with previous omicron subvariant BA.1 or BA.2, as explained by a study published in the journal Cell. 

Back in mid-July, White House COVID-19 Response Coordinator Dr. Ashish Jha said BA.5 is both the "most contagious" and "most immune evasive variant we've seen" in an interview with NBC's Andrea Mitchell.

Unfortunately, researchers are confident that re-infections with BA.5 are likely even if you've been infected with previous omicron subvariant BA.1 or BA.2, as explained by a study published in the journal Cell. A separate study published in the New England Journal of Medicine found that existing immunity against COVID-19 has a harder time defeating BA.5 compared to early omicron subvariants. As Salon previously reported, this means re-infections are likely.

According to the Centers for Disease Control and Prevention (CDC), studies suggest that reinfection with the same virus variant as the first infection — or, reinfection with a different variant — are both possible. Reinfections can happen within a mere 90 days of the initial infection. A specific report that identified 10 people who got reinfected found that they occurred between 23 to 87 days after initial infection.

But experts widely agree that despite the virus mutating to evade immunity, that doesn't mean you aren't building some immunity through infection or vaccination. In other words, researchers believe that previous infections or vaccinations among patients means that many cases have remained relatively mild.

The second major question around BA.5 involves how severe it is. Different variants and subvariants seem to cause different degrees of disease severity — and even different symptoms. So does BA.5 cause more severe disease, especially in those who haven't been vaccinated?

Currently, there is no evidence to suggest BA.5 is causing more severe disease due to its mutations. However, hospitalization numbers have increased in the past few months. Yet experts don't believe that rise is a result of the variant itself, but rather the rise in infections which will inevitably lead to more hospitalizations, as reported by NBC News.

Experts also caution that vaccination provides protection, especially against severe disease and hospitalization. As NPR reported, unvaccinated people are five times more likely to get infected and 7.5 times more likely to be hospitalized than those who are vaccinated.

"Let me make a clear, clear point here that's a little tough to hear: Whether you've been vaccinated, whether you've been previously infected, whether you've been previously infected and vaccinated, you have very little protection against BA.5 in terms of getting infected or having mild to moderate infection," Dr. Gregory Poland, head of the Mayo Clinic's Vaccine Research Group said. "You have good protection against dying, being hospitalized or ending up on a ventilator."

As Salon previously reported, omicron-targeted boosters could be here as soon as this fall. But will they come too late, after another variant has already emerged? It's too early to tell.

L.J. Tan, chief strategy officer for the Immunization Action Coalition, previously told Salon that it is "unlikely" that in the fall there will be another variant that is so dramatically different that potential new boosters wouldn't make some sort of an impact.

"I don't think that it's going to drift so much that a vaccine that contains either of those existing omicron epitopes is going to not work," Tan said. The word "epitope" means the substance that the immune system recognizes and protects against. "So for that reason, I don't think that we're going to get this horrible strain that's going to evade everybody's vaccine response, and that's just because of the way the virus evolves," Tan continued.

By Nicole Karlis

Nicole Karlis is a senior writer at Salon, specializing in health and science. Tweet her @nicolekarlis.

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