With new coronavirus sub-variants emerging, the World Health Organization (WHO) is concerned that countries are testing less.
"We need to follow this virus closely in every single country," said WHO Emergency Director Mike Ryan in Geneva on Wednesday.
The virus is constantly changing, new developments must be discovered as early as possible.
"We cannot afford to lose sight of the virus." It would be very short-sighted to think that the risk of contagion has decreased because of fewer reported infections.
The WHO still lists Delta and Omicron as "variants of concern".
At Omikron, this includes several lines, including the recently emerged BA.4 and BA.5.
They have been detected in South Africa and in some European countries, said WHO Covid-19 expert Maria van Kerkhove.
Both showed partly different characteristics than other omicron variants.
Specialists are investigating whether BA.4 and BA.5 spread faster than other virus lines, whether they differ from others in terms of the course of the disease and how vaccines work against them.
So far there is no evidence that people infected with BA.4 or BA.5 have a more severe course of the disease, said van Kerkhove.
According to their information, however, fewer than 200 sequencings of these subvariants have been uploaded to the WHO database.
Van Kerkhove called on countries to continue to closely monitor the development of variants.
The number of deaths reported each week is falling worldwide, WHO chief Tedros Adhanom Ghebreyesus said.
On April 10, 22,000 deaths were reported in seven days, the lowest since the early weeks of the 2020 pandemic.