Omicron subvariant BA.2 continues global rise as experts assess mixed data

A sub-lineage of the omicron coronavirus variant, dubbed BA.2, continues to increase steadily around the globe as scientists and health officials are still working to understand the risk it poses to public health.

So far, the overall data has been a mix. Some recent laboratory and animal data have suggested that BA.2 can cause more severe disease than the original omicron variant, BA.1. But, so far, that finding isn't bearing out in real-world data. Countries where BA.2 is dominant are not seeing higher rates of severe disease. And, many places seeing BA.2 increasing are also seeing cases decline, along with hospitalizations.

While animal experiments have hinted that BA.2 interacts differently to some immune responses than the original omicron variant, so far real-world vaccine data finds two doses and booster doses are just as effective—if not slightly more effective—against BA.2 than BA.1.

There is one thing that everyone agrees on, however: BA.2 is a little more transmissible than BA.1, which was already considered ultratransmissible. Studies have consistently found that BA.2 has a growth advantage, and current estimates peg BA.2's transmission as about 30 percent to 40 percent higher than BA.1's. That explains how BA.2 is now chipping away at BA.1's global domination.

BA.2 now accounts for at least 21 percent of all sequenced omicron cases worldwide. It has overtaken BA.1 as the dominant virus in at least 10 countries, including Bangladesh, China, Denmark, India, Nepal, Pakistan, and the Philippines. Where it rises, it rises quickly. In South Africa, for instance, BA.2 jumped from 27 percent on February 4 to 86 percent by February 11. In the United Kingdom, BA.2 prevalence jumped six-fold from January 17 to January 31. And in the US, it has more than tripled from 1.2 percent in the week ending on January 29 to its most recent prevalence estimate of 3.9 percent as of February 12.

Mostly good news

But those rises haven't been accompanied by concerning upticks in severe disease and hospitalizations, as noted in a recent epidemiological report by the World Health Organization. In Denmark, where BA.2 is quickly nearing 100 percent of all infections, researchers have seen no difference in hospitalizations among people infected with BA.2 compared with BA.1. The analysis accounted for sex, age, vaccination status, time period, region, comorbidity, and previous SARS-CoV-2 infection. In South Africa, where BA.2 is also dominant, hospital admissions continue to decline. And likewise, in Nepal, though BA.2 cases have risen in February, cases still continue to fall from late January, and use of intensive care and mechanical ventilation is also on the decline.

Together, that data is comforting given a recent pre-print study suggesting that BA.2 appears to be more pathogenic than BA.1—at least in lab and animal experiments. The study, led by researchers at the University of Tokyo, found that BA.2 could bind to human cells better than BA.1 and replicated to higher levels in lung and nasal cells. In experiments with hamsters, the researchers also found that BA.2 caused more severe lung disease than BA.1. Work with hamsters and mice also suggested that BA.2 could thwart immune responses generated to BA.1. But this finding didn't hold up statistically when the researchers pitted BA.2 against antibody samples from three unvaccinated people who had recovered from BA.1. The rodent data also conflicts with the real-world data from Denmark, referenced above.

Vaccine-effectiveness data from the UK and Denmark offer yet more comfort. A recent report released by the UK Health Security Agency found that current vaccines are just as effective—if not slightly more effective—against BA.2 than BA.1. Specifically, 25 weeks after a second dose, vaccines were 10 percent effective against symptomatic COVID-19 from BA.1 but were 18 percent effective against BA.2. Protection against symptomatic infection from BA.1 increased to 69 percent two weeks after a booster, but protection increased to 74 percent against BA.2. Preliminary data from Denmark, noted in the WHO report, found that vaccinated people with breakthrough BA.2 infections were less likely to spread the infection to household contacts than vaccinated people infected with BA.1

Overall, the WHO concluded that this "suggests that vaccination is at least equally effective in preventing acquisition of BA.2 and could be more effective in preventing transmission of BA.2 compared to BA.1."