Two new fast-spreading subvariants of Omicron are causing fresh surges of Covid around the world.
BA.4 and BA.5 were first identified in South Africa and may soon become the dominant strains in Europe and the US, health experts say.
What exactly are BA.4 and BA.5?
Ever since it first emerged, Covid has been mutating or shape-shifting. The new genetic versions that keep appearing are called variants.
There have been a few major variants already, such as alpha and delta, that have caused massive waves of infection.
The latest ones experts are concerned about - BA.4 and BA.5 - are very closely related to the Omicron variant behind last winter's wave.
They were added to the World Health Organization's monitoring list in March and have also been designated as variants of concern in Europe.
Where are they spreading?
They were spotted circulating in South Africa at the beginning of the year and now appear to be spreading much more quickly than other variants.
Most European countries now have them and they look set to overtake other types of Covid soon. That's already happened in Portugal - BA.5 is now dominant there.
In the US, officials say they are seeing rising numbers of infections caused by the two new subvariants.
Covid infections in the UK are also showing early signs of a possible rise, driven by BA.4 and BA.5.
Australia has reported cases too.
Will they be harmful?
Experts are unsure how hard countries will be hit.
BA.4 and BA.5 Omicron are not thought to be any more lethal than other types of Covid.
Lots of people have built up some immunity from past infections and vaccination, which is helping to make the disease less risky overall.
But the new subvariants do appear to be spreading more easily.
This is partly because immunity may be waning, but also because of the mutations the virus has undergone.
Many countries have also lifted their Covid restrictions, meaning people are mixing more, which gives the virus more chances to spread.
BA.4 and BA.5 appear to be able to infect people even if they've recently had other types of Omicron.
A wave of new infections could lead to more hospitalisations and some more deaths.
How can we protect against them?
As with other Covid variants, the risk or serious illness remains highest for people who are elderly, or who have significant underlying health conditions.
Although current vaccines are not a perfect fit, they are still the best line of defence.
They have cut the risk of severe illness against the other major Covid variants, including Delta, Alpha, Beta and Gamma.
Doctors say it is vital people get the recommended number of doses to gain maximum protection against existing and emerging variants.
How quickly could we get new vaccines against variants?
Updated versions of vaccines against Covid variants are already being designed and tested.
Manufacturers could scale up production quickly too, and regulators have already discussed how to fast-track the approval process.
Why do variants occur?
Viruses make carbon copies of themselves to reproduce, but they aren't perfect at it. Errors creep in that change the genetic blueprint, resulting in a new version of the virus.
If this gives the virus a survival advantage, the new version will thrive.
The more chances coronavirus has to make copies of itself in us - the host - the more opportunities there are for mutations to occur.