Stealth Omicron Variant BA.2 1.5 Times More Contagious Than Original but Vaccines Still Work

The Omicron BA.2 COVID-19 sub-type may be 1.5 times more contagious than the previous BA.1 type, according to new data from Denmark—but booster shots work against it.

The country has been one of the worst-affected in Europe by the BA.2 sub-type which has been responsible for an increasing proportion of COVID-19 cases in a number of nations recently.

As of January 26, BA.1 was still the most common Omicron sub-variant worldwide, accounting for 98 percent of cases at that time, according to Denmark's Statens Serum Institut (SSI) infectious diseases organization.

But in Denmark itself, BA.2 developed quickly and has become the country's dominant COVID type.

In a press release on Wednesday, the SSI said BA.2 appeared to be one and a half times more contagious than BA.1, based on preliminary calculations.

Dr. Tyra Grove Krause, an infection epidemiology expert at the organization, stated: "Of course, we follow the development closely, and if BA.2 is more contagious, it may mean that the wave of infections will be higher and will extend further into February compared to the previous projections."

BA.2 had been reported from at least 54 countries worldwide, including the U.S. as of January 27, according to Outbreak.Info which uses data from the virus genetic database GISAID.

While experts shed more light on how infectious BA.2 might be, data has also suggested that vaccines are still effective against it.

On Thursday the U.K.'s Health Security Agency (HSA) reported that vaccine effectiveness against symptomatic disease with BA.2 compared to BA.1 had recently been analyzed. The results showed that vaccine effectiveness against symptomatic disease was similar for BA.1 and BA.2.

The early data showed that, two weeks after a booster dose, vaccine effectiveness was around 63 percent for BA.1 and 70 percent for BA.2. Effectiveness was significantly lower—between 9 and 13 percent—in those who had only had two doses after 25 weeks. The data refers to all vaccine brands combined.

That same day Dr Mary Ramsay, Head of Immunisation at HSA, said in a press release: "The evidence is clear—the vaccine helps to protect us all against the effects of COVID-19 and the booster is offering high levels of protection from hospitalization and death in the most vulnerable members of our society."

The U.K. vaccines minister Maggie Throup called boosters "absolutely crucial" for increasing immunity against Omicron.

The BA.2 variant has sometimes been referred to as the "stealth" variant of Omicron due to a change in the way it is detected compared to BA.1. However, the variant will still result in a positive COVID-19 test.

COVID vaccine
A COVID vaccine being given at a healthcare facility in Chile in December, 2020. Vaccine booster shots are as effective against symptomatic disease with BA.2 as BA.1, early U.K. data suggested. Marcelo Hernandez/Getty