Covid-19 cases are rising again in the US, driven by the growth in BA.2

According to the latest estimates from the US Centers for Disease Control and Prevention, BA.2 caused 86% of the new Covid-19 cases nationwide last week.

In some ways, it feels like a familiar place. The cases go again. At least one major city is reinstating its mask mandate. Broadway shows have canceled some performances.

But there is still cause for optimism.

Despite BA.2’s almost complete takeover of two other circulating Omicron sub-variants, BA.1 and BA 1.1, US hospital admissions are at record low levels and they continue to decline. Deaths also continue to fall.

Although these numbers tend to lag behind the number of cases, the United States has not seen a steep increase in infections. Whether that is likely to happen is still an open question.

Even Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, does not know what BA.2 will do. As a nation, transmission must go down to a level that is “low enough that it does not disrupt our population or economy, our daily economic, workplace and social life, which means it must be low enough that it is not a serious threat to the nation’s health, “he says. He does not know if we are out of the woods.

“We are certainly seeing the beginning of a wave of new infections,” Fauci said. “It depends on how high we go up in the increase, and it depends on whether the increase is associated with an increase in serious illness.

“I can not say where we are right now because we are changing,” he said.

Cases are rising regionally

Across the nation, Covid-19 cases have increased by 24% from where they were two weeks ago, and the United States now has an average of about 38,000 cases a day. This is a jump from last week, probably because Florida recently reported a two-week backlog. Still, it is one of the lowest daily prices since July.

State by state, however, the picture is more mixed. Cases rise in 25 states, fall in 16 and remain stable in nine others.

Cases are rising fastest in the Northeast, the region of the United States that has the most BA.2 transmission.

On Monday, Philadelphia became the first major city in the United States to announce a return to indoor mask requirements. Cases have risen 50% there over the last 10 days, which has pushed the city above the threshold that triggers the mask mandates.

“I suppose this wave will be smaller than the one we saw in January,” said Dr. Cheryl Bettigole, Philadelphia’s Public Health Commissioner, Monday.

“But if we wait to find out and put on our masks again, we’ve lost our chance to stop the wave.”

Several universities, including Johns Hopkins, American, George Washington and Georgetown, have also reintroduced indoor masking.

New York City had reconsidered its mask requirements for preschoolers, but with increasing cases in the city, Mayor Eric Adams recently said masks would continue to be required for the youngest children, who had higher rates of hospitalizations under Omicron than in previous waves.

In addition to the number of cases, which can be a less reliable pandemic measure because the number of tests has dropped, coronavirus levels in wastewater tell a largely reassuring story.

Wastewater monitoring is considered a reliable warning of what is coming. U.S. numbers are rising slightly, but are still at one of the lowest levels seen since July, according to Biobot Analytics, a company that analyzes wastewater samples from across the country.

Different countries, different stories

The situation with BA.2 here seems to be a departure from that seen in the UK and Europe.

According to variant-tracking site, the Netherlands was near the top of its BA.2 wave as the subvariant reached 83% of infections there in the second week of March. Switzerland was also close to its BA.2 peak when the subvariant reached 80% of infections in mid-March. After falling for weeks, cases in the UK had doubled from a low point on 25 February and would soon reach the height of the BA.2 wave, as the sub-variant caused 88% of cases there between 7 March and 21 March.

The BA.2 experience in the United States is much more similar to what happened in South Africa. In the second and third weeks of February, when BA.2 was responsible for about 88% of the transmission there, the cases got a little bump, but then continued to fall during the month of March.

“I have been cautiously optimistic about BA.2 because of the trends that there has not been such an exponential increase in cases as we saw when Omicron first appeared,” said Pavitra Roychoudhury, who studies the spread of infectious diseases. at the University of Washington School of Medicine.

She said the tsunami of Omicron that hit the United States over the winter has left a lot of immunity in the wake. We are also more vaccinated and boosted as a country than we have ever been – even though the health authorities say we could do much better with boosters.

Some listen to that warning. The vaccination rate has roughly doubled over the last two weeks as more people seek out other boosters.

An average of about 502,000 vaccine doses have been administered daily over the past week, according to CDC data. That’s up from about 219,000 doses a day on March 29, when the CDC and US Food and Drug Administration approved another booster shot for people 50 years and older, though the CDC does not specifically count another booster.

“That might explain our somewhat more optimistic outlook compared to places like the UK, where there was a significant increase and it was associated with BA.2,” Roychoudhury said.

BA.2 in the UK

In general, during the pandemic, health officials have pointed to Britain as a warning of things to come in the United States, but that kind of extrapolation can become harder to do as populations develop different types and degrees of immunity.

After a wave of cases caused by Omicron’s BA.1 subvariant peaking in January and then declining, the UK saw another increase in cases and hospitalizations with BA.2. That wave peaked in late March, and since then, things have been on a steep decline.

Adam Kucharski, an epidemiologist at the London School of Hygiene & Tropical Medicine who tracks outbreaks of infectious diseases, believes the BA.2 wave in the UK was at least partly due to the timing of its booster campaign.

The UK started offering booster shots or third vaccine dose in mid-September, just a few days before the US did. But more people got them: In the UK, 68% of people over the age of 12 who are eligible for a booster dose have received one; in the United States, that figure is only 45%, according to the CDC.

Many people in the UK who received a booster in September or October still had high antibody protection when Omicron arrived.

Antibodies are the first line of defense in an infection. They act quickly to limit the spread of a virus through the body. Antibodies are highest in the first few months after vaccination and decrease over time. But even after they fall off, the body retains its immune memory to vaccines and can quickly upgrade to do more if it is infected.

Omicron was identified in late November, with many in the UK still within the window of highest protection against their booster doses.

“I think we were lucky that the boosters, at least in the short term, provided quite a lot of protection,” Kucharski said.

People who had recently had boosters had such good immunity that even if they were infected with BA.1, they might not have known it because their symptoms were so mild. It was likely that they did not contribute to the transmission, Kucharski believes, so boosters did a good job of keeping Omicron’s violent spread through the UK over the winter.

But fast forward three months and many people who had been given boosters as recommended were six months past their shots. Studies show that antibody levels drop four or five months after the third dose, so their protection against infection was probably much lower just as BA.2 arrived at the site.

And now, says Kucharski, with BA.2, even the boosted group began to get “mild, mild symptoms or enough to detect it and test positive” and be counted as a coincidence.

As immunity waned, BA.2 snuck in

Whether the U.S. will see a new wave of cases from BA.2 will depend a lot on two things, Kucharski said: the current level of immunity in the population and our behavior.

About half of those eligible for a booster dose in the United States have had one, and several million became infected with Omicron, which estimates that nearly 95% of Americans, according to the CDC, have some degree of protection against coronavirus.

However, for those who have lost their immunity over time because protection against their original two-dose vaccinations has diminished, or because they became infected a year or more ago with an older variant, the virus may find a way to spread. .

“I think if BA.2 can find susceptibility, which will translate into growth in cases, “Kucharski said.

But he stresses that much will depend on what Americans are doing right now.

“I think the question is what happens in the meantime, if actual booster campaigns and other things are in line, then it can offset [the subvariant]. But I think, based on what we see in many countries in Europe, if there is susceptibility, either because people have not been given a booster or they had it a while ago, then it could result in a growing epidemic. ,” he said.

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