It has been a rather busy couple of months for the World Health Organization (WHO), more so than the rest of the year, as the omicron variant of COVID-19 spiked and spread across the world. It seems even more busy times are ahead as the WHO said it was monitoring and analyzing a few dozen cases of two new sub-variants of the highly transmissible omicron strain to assess whether they are more infectious or dangerous.

It has added BA.4 and BA.5, sister variants of the original BA.1 omicron variant, to its list for monitoring. It is already tracking BA.1 and BA.2 – now globally dominant – as well as BA.1.1 and BA.3.

The WHO said it had begun tracking them because of their "additional mutations that need to be further studied to understand their impact on immune escape potential."

Viruses mutate all the time but only some mutations affect their ability to spread or evade prior immunity from vaccination or infection, or the severity of disease they cause.

For instance, BA.2 now represents nearly 94% of all sequenced cases and is more transmissible than its siblings, but the evidence so far suggests it is no more likely to cause severe disease.

Since it was first identified in November, BA.2 has been spreading around the globe, driving new surges in parts of Asia and Europe. It's now the dominant coronavirus version in the U.S. and more than five dozen other countries.

It was given the "stealth" nickname because it looks like the earlier delta variant on certain PCR tests, says Kristen Coleman at the University of Maryland School of Public Health. The original omicron, by contrast, is easy to differentiate from delta because of a genetic quirk.

In rare cases, early research indicates BA.2 can infect people even if they've already had an omicron infection. COVID-19 vaccines appear just as effective against both kinds of omicron, offering strong protection against severe illness and death.

Only a few dozen cases of BA.4 and BA.5 have been reported to the global GISAID database, according to WHO.

The U.K.'s Health Security Agency said last week BA.4 had been found in South Africa, Denmark, Botswana, Scotland and England from Jan. 10 to March 30.

All the BA.5 cases were in South Africa as of last week, but on Monday Botswana's health ministry said it had identified four cases of BA.4 and BA.5, all among people aged 30 to 50 who were fully vaccinated and experiencing mild symptoms.