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Omicron subvariants BA.4 and BA.5 have been detected in more than a dozen countries driving Covid outbreaks across the world, according to the World Health Organization.

According to WHO's technical lead on Covid Maria Van Kerkhove, at least 16 countries have reported about 700 cases of BA.4, while more than 300 cases of BA.5 have been found across at least 17 countries, CNBC reported.

The new sub-variants do not cause severe infection than the original Omicron strain, but they appear to be more contagious, Van Kerkhove was quoted as saying during a Q&A on the organisation's social media platforms on Wednesday.

She noted the WHO is monitoring BA.4 and BA.5 to determine if they will eventually overtake BA.2 as the dominant strain worldwide.

"We don't know how this variant will behave, how these sub-variants will behave in other countries that had a dominant wave of BA.2," Van Kerhkove said. "This is what remains to be seen."

A recent study, not yet peer-reviewed, showed that BA.4 and BA.5 have the ability to avoid immunity induced by both previous Covid infection and vaccination. The findings by researchers from the Africa Health Research Institute in South Africa could signal a fresh wave of infections by the BA.4 and BA.5.

Last week, a report by the UK Health Security Agency showed that South Africa reported the highest - 395 cases of BA.4 and 134 cases of BA.5 as of May 6.

Other countries that detected the variants include Austria, UK, US, Denmark. Belgium, Israel, Germany, Italy, Canada, France, the Netherlands, Australia, Switzerland, Botswana, Portugal,A Belgium, Hong Kong, Canada, Israel, Norway, Pakistan, Spain and Switzerland, the report said.

The report noted the number of sequences is low, but "the apparent geographic spread suggests that the variant is transmitting successfully."

Van Kerkhove also mentioned another Omicron subvariant called BA.2.12.1, which has been detected in 23 countries. She said there are more than 9,000 reported sequences of the subvariant, most of which comes from the US, the report said.BA.2.12.1 made up about 42.6 per cent of all new cases in the U.S. during the week that ended on May 7, according to data from the Centres for Disease Control and Prevention.

Van Kerkhove said that the higher growth rate of BA.2.12.1 can likely lead to increased transmission globally, but it has shown no difference in hospitalisation rates in comparison to BA.2.

She urged governments across the world to closely monitor BA.2.12.1, BA.4, BA.5 and other sub-variants that could emerge in the future, emphasising the need to maintain Covid testing and sequencing.