A doctor monitors a Covid-19 patient in the Covid-19 intensive care unit of the community hospital in Germany on April 28, 2021.RONNY HARTMANN | AFP | Getty ImagesLONDON — Covid cases are rising in Europe, with an increasing number being attributed to the prevalence of a “stealth” subvariant of the omicron strain.Covid cases have risen dramatically in the U.K. in recent weeks, while Germany continues to mark record high daily infections with over 250,000 new cases a day. Elsewhere, France, Switzerland, Italy and the Netherlands are also seeing Covid infections start to rise again, aided and abetted by the relaxation of Covid measures and the spread of a new subvariant of omicron, known as BA.2.Public health officials and scientists are closely monitoring BA.2, which has been described as a “stealth” variant because it has genetic mutations that could make it harder to distinguish from the delta variant using PCR tests, compared to the original omicron variant, BA.1.The new subvariant would be the latest in a long line to emerge since the pandemic began in China in late 2019. The omicron variant — the most transmissible strain so far — overtook the delta variant, which itself supplanted the alpha variant — and even this was not the original strain of the virus.Now, Danish scientists believe that the BA.2 subvariant is 1.5 times more transmissible than the original omicron strain, and is already overtaking it. The BA.2 variant is now responsible for over half of the new cases in Germany and makes up around 11% of cases in the U.S.That number is expected to rise further, as it has in Europe.”It’s clear that BA.2 is more transmissible than BA.1 and this, combined with the relaxation of mitigation measures and waning immunity, is contributing to the current surge in infections,” Lawrence Young, a professor of molecular oncology at Warwick University, told CNBC Monday.”The increased infectiousness of BA.2 is already out-competing and replacing BA.1, and we are likely to see similar waves of infection as other variants enter the population.”As long as the virus continues to spread and replicate, particularly in populations which are under-vaccinated or where vaccine-induced immunity is decaying, “it will throw up new variants and these will remain a continual threat even to those countries with high rates of vaccination,” Young noted. “Living safely with Covid doesn’t mean ignoring the virus and hoping it will go away forever.”What do we know about BA.2?The BA.2 variant is being closely monitored by the World Health Organization and similar public health bodies on a national level, including the U.K. Health Security Agency (UKHSA) which has said the subvariant is “under investigation” but is not yet of concern.Still, the WHO acknowledged in a statement last month that “the proportion of reported sequences designated BA.2 has been increasing relative to BA.1 in recent weeks.”Initial data show that BA.2 is a little more likely to cause infections in household contacts, when compared with BA.1. It’s not believed currently that the BA.2 variant causes more severe illness or carries an increased the risk of being hospitalised, however further research is needed to confirm this, according to a U.K. parliamentary report published last week.Hospitalizations have also risen in a number of European countries as Covid infections have risen in recent weeks, but deaths remain far lower than in previous peaks thanks to widespread vaccine coverage.The UKHSA has done a preliminary analysis comparing vaccine effectiveness against symptomatic disease for BA.1 and BA.2 infections and found that the levels of protection are similar, with efficacy of up to 77% soon after a booster shot, although this wanes over time.’Growth advantage’The WHO has also noted that BA.2 differs from BA.1 in its genetic sequence, including some amino acid differences in the spike protein and other proteins which could give it an advantage over the original omicron.”Studies are ongoing to understand the reasons for this growth advantage, but initial data suggest that BA.2 appears inherently more transmissible than BA.1, which currently remains the most common omicron sublineage reported. This difference in transmissibility appears to be much smaller than, for example, the difference between BA.1 and Delta,” the WHO said last month.The WHO added that initial studies suggest that anyone who has been infected with the original omicron variant has strong protection against reinfection with its subvariant BA.2.Dr Andrew Freedman, reader in infectious disease at Cardiff Medical School, told CNBC he doesn’t think we need to be too concerned about BA.2, despite the fact that it is slightly more contagious.”I suspect the rising number of cases is related to several factors including BA.2, the relaxation of restrictions and more social mixing, less mask wearing and some waning of immunity from both previous infections and vaccination, especially in those who received boosters early on,” he said.”There has been an upturn in hospital admissions testing positive for Covid in the U.K., but many of these are incidental and there has not been a parallel increase in deaths.”Growing in prevalenceThe U.K., and the rest of Europe, have acted as a bellweather for the U.S. at several points in the pandemic, declared two years ago by the WHO. particularly when it comes to the rise and spread of new Covid variants which have emerged and subsequently supplanted previous strains of the virus.This makes the emergence and growing prevalence of the BA.2 variant a point of concern for the U.S. where cases have nosedived recently to reassuring lows.Already some parts of the U.S. are seeing an increasing number of infections linked to BA.2, particularly in New York, according to data collected by the U.S. Centers for Disease Control and Prevention.Elsewhere, China is currently facing its worst Covid-19 outbreak since the height of the pandemic in 2020.It’s unclear whether BA.2 is contributing to the latest wave of cases, although a prominent infectious disease expert in China told news outlet Caixin that much of the current outbreak is being driven by the BA.2 subvariant.U.K. data certainly illustrates BA.2’s increasingly prevalence. Sequenced data from Feb. 27 to Mar. 6 found that 68.6% of cases were omicron lineage BA.2, with just 31.1% omicron BA.1.