The study reveals the likely reason for the recent success of “stealth” Omicron


The ability to spread more easily from person to person appears to be the superpower propelling an up-and-coming sibling of the Omicron variant into widespread distribution, a group of scientists have suggested.

This conclusion about the virus known as the “Stealth Omicron” is explained in a brief report published this week by the New England Journal of Medicine. It is based on the team’s finding that the virus, whose official name is BA.2, is only marginally better than the dominant BA.1 at bypassing the protective effects of COVID-19 vaccines.

Both “subvariants” of Omicron share a common core of genetic mutations. But everyone has a few that the other doesn’t have. For example, BA.2 lacks a mutation that belongs to other versions of Omicron that makes it easy to tell it apart from the Delta variant. (Hence the nickname “Stealth”.)

Epidemiologists can watch BA.1 and BA.2 compete in a race to infect humans and tell you who wins. But scientists studying the evolution of viruses want to understand not just what wins, but why. By gaining more insight into how single mutations alter a virus’ behavior, they can be better prepared for its next genetic change.

Currently, BA.2 is making a solid run against BA.1 in the UK and could be linked to the resurgence of COVID-19 there and in other European countries.

BA.1 is still the dominant coronavirus in the United States and has been since early January. But BA.2 now accounts for 23.1% of new coronavirus infections here, according to the Centers for Disease Control and Prevention estimates. A week earlier it accounted for 13.7% of US infections and the week before just 7.1%.

With scientists’ growing understanding of the behavior of the SARS-CoV-2 coronavirus, as well as the locations of the subvariant mutations, the list of potential attributes that could account for its success has been narrowed down to two main suspects: immune evasion and transmissibility.

If the virus got better at evading the protection afforded by either natural infections or vaccines, it could increase its numbers by infecting people thought to be immune. And if this is the case, more comprehensive vaccinations, additional booster shots or reformulated vaccines may be required.

But if that’s not the case, the best alternative explanation is that the virus has improved its ability to spread, and it could do so in many ways. Perhaps it causes more asymptomatic infections, allowing infected people to keep circulating and infecting others. Perhaps the virus will stay in the air longer and get more chances to find new hosts. Perhaps it establishes itself more easily in the nose and mouth, where it is more likely to be expelled into the path of new victims.

To find out, scientists at the Massachusetts Consortium on Pathogen Readiness set out to investigate whether BA.2 would be better suited to infecting people who should have had some immunity, either from a COVID-19 vaccine or a previous coronavirus -Infection.

Omicron is known to have found ways to evade the immune defenses of vaccinated people more easily than the original coronavirus, which originated in Wuhan, China, and the later Alpha and Delta variants. Omicron has also turned the effectiveness of COVID-19 treatments on their head, which worked by mimicking the immune system.

Researchers investigated whether subvariant BA.2 further enhanced BA.1’s ability to evade the vaccine. They recruited 24 healthy people and drew blood samples from them three times: after receiving two doses of Pfizer’s BioNTech vaccine; six months later, when it was known that immunity had waned, and two weeks after they received a booster shot.

In a lab, they tested these subjects’ plasma against the original Wuhan strain of the virus, as well as BA.1 and BA.2. Then they measured the neutralizing antibody response.

The authors found that after two and three doses of vaccine, BA.2 was slightly better than BA.1 at overcoming the immune system of vaccinated individuals. In subjects who received three doses of the vaccine, levels of neutralizing antibodies – the immune proteins that thwart infection – were about 40% higher against BA.1 than against BA.2.

That might sound like a big difference, but it actually isn’t, said Dr. Dan Barouch of Beth Israel Deaconess Medical Center, one of the authors of the new report.

“These are very different from the changes we saw between the Alpha and Delta variants, or when Omicron overtook Delta,” Barouch said. “These were 20- and 40-fold differences. We were really talking about orders of magnitude difference in neutralizing antibodies.”

In this case, the smaller difference in antibody levels between BA.1 and BA.2 means that over the next week you would find if you sent a large group of vaccinated and boosted people to a party where both subvariants were circulating were that BA.2 infected a few more people than BA.1. But if you exposed partygoers to the Wuhan virus and Omicron, the Omicron variant would likely have caused virtually all infections.

The good news, Barouch added, is that BA.2 didn’t really boost Omicron’s ability to infect vaccinated humans. The bad news is that both BA.1 and BA.2 are already pretty good at it, he said.

There’s some other pretty good news, although it’s very preliminary. In a test involving eight people recently infected with Omicron (and given the timing, BA.1 was thought to be the culprit), most of them had high levels of antibodies against the BA.2 variant. Only one person – an unvaccinated subject who had recently been infected – lacked the antibodies needed to prevent reinfection.

This finding suggests that for the many vaccinated Americans who became infected during the omicron wave, this stealth omicron is unlikely to spark a new wave of breakthrough cases.

The scientists wrote that BA.2’s apparent ability to sideline BA.1 “is probably related to increased transmissibility rather than enhanced immunological escape.”

This is the beginning of the investigation, not the end. From here, scientists want to examine exactly how BA.2 managed to make itself even more transmissible than the already wildly contagious BA.1. The study reveals the likely reason for the recent success of “stealth” Omicron

Tom Vazquez

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