Two years into the pandemic, a mutated version of the omicron variant, known as BA.2, has become the latest challenge to taming COVID-19.

The subvariant, detected in at least 57 countries, appears to spread even more easily than the original. But so far it doesn't seem to cause more severe disease and booster shots remain an effective shield. Scientists are racing to answer a number of questions about this variant as they prepare for the next one.

How transmissible is BA.2?

Omicron is more transmissible than delta, the variant that became globally dominant in the middle of last year, and the new subvariant is especially infectious. Take Denmark, where BA.2 has quickly gained ground. A study based on about 8,500 households in December and January found that people infected with BA.2 spread the virus to an average of 39% of susceptible household members, versus 29% for the original omicron variant. That's in line with preliminary U.K. data.

Is BA.2 more dangerous than Omicron?

Omicron and its related strains appear less likely than earlier variants to cause serious illness, particularly in vaccinated people. The World Health Organization said this week that BA.2 doesn't seem more severe than the original omicron variant, based on evidence from Denmark, where its spread hadn't led to unexpected spikes in hospitalizations. In announcing the end of coronavirus restrictions late last month, the Danish government declared the disease no longer poses a threat to society, even as cases hit a record high.

How well do vaccines work against it?

The data suggest some Covid vaccines are less effective at preventing infections caused by omicron than previous variants, while still doing a good job protecting against severe disease. Although the subvariant appears to be even more contagious, Covid shots -- in particular booster doses -- are just as effective against BA.2, according to initial findings from U.K. health authorities. By 25 weeks or more after the second dose, vaccines blocked 13% of cases -- a rate that increased to 70% two weeks after a booster.

How does BA.2 differ from the original?

The latest iteration of the coronavirus represents a small minority of infections but has rapidly spread in countries such as South Africa, Denmark, India and England. The two versions differ by some 40 mutations, including a key alteration in the spike region of BA.2. While the two are related, there are enough differences to drive a change in behavior. The milder form of most omicron cases in vaccinated people may leave those who recover still vulnerable to existing virus and future variants, according to researchers.

What does it mean for the pandemic?

Virus hunters are trying to better understand the properties of BA.2 as they brace for future variants, and some believe BA.2 could prolong the omicron wave. The characteristics of the subvariant may lead to a "substantially longer tail of circulation of omicron," computational biologist Trevor Bedford of the Fred Hutchinson Cancer Research Center said in a Jan. 28 Twitter post. A potentially slower decline in cases could lead to higher hospitalizations and pose a problem for countries with lower vaccination rates.