"Many have themselves become infected and, while reporting is scant, we estimate that at least 115,000 health and care workers have paid the ultimate price in the service of others," UN health chief Tedros Adhanom Ghebreyesus said on Monday.

Tedros said he would not ask for a minute's silence and instead called on the few dozen people present in the meeting room as well as everyone tuning in online from capitals around the world to give thanks for the millions of health care workers worldwide.

He encouraged those watching to clap, cheer and stamp their feet, himself joining in on a lengthy round of applause, echoing health care tributes made in cities around the world at the start of the pandemic when citizens went outside to clap at certain times of day.

Tedros also criticised rich countries for buying up most of the available vaccine doses.

He said 75 per cent of the vaccine doses were in just 10 countries.

In many other countries, millions of health workers would therefore have to wait to be vaccinated.

He appealed to wealthier countries to give up their doses instead of providing jabs to children and called on health care officials to ensure at least 10 per cent of people worldwide can be vaccinated by September and 30 per cent by the end of the year.

"I understand that every government has a duty to protect its own people. I understand that every government wants to vaccinate its entire population," he said.

"But right now, there is not enough supply. Countries that vaccinate children and other low-risk groups now do so at the expense of health workers and high-risk groups in other countries - that's the reality."

Tedros added that the pandemic was on track to claim more lives by mid-June this year than all of 2020.

The number of infections reported to the WHO in the first five months of 2021 have already eclipsed the total number registered in the first year of the pandemic.

"We are pleased that for three consecutive weeks we have seen a downturn in the number of cases and deaths being reported. But globally, we remain in a fragile situation," Tedros said.

"No country should assume it is out of the woods, no matter its vaccination rate."

He said that although the currently available COVID-19 vaccines and therapeutics were effective against the new coronavirus variants detected by scientists, it would not necessarily remain the case.

"This virus is changing constantly. Future changes could render our tools ineffective and drag us back to square one. We must be very clear: the pandemic is not over and it will not be over until and unless transmission is controlled in every last country."

In a video message played at the assembly, German Chancellor Angela Merkel issued a warning about future pandemics, telling officials that the scientific community needed to be better prepared.

"This pandemic has not yet been overcome and it will not be the last," Merkel said.

She called for an international pandemic treaty allowing easier collaboration between countries and a faster warning system so as to stop the next global pandemic in in its tracks.

"After the pandemic is before the pandemic," Merkel said, echoing a famous forward-looking quote from West Germany football manager Sepp Herberger ("After the match is before the match").

"We should be as well prepared as possible for the next one. That is the response I hope to get from this World Health Assembly."

The World Health Assembly (WHA) meets every year to set health care priorities and decide on budget matters, among other things.

It kicked off an online version of its annual summit in Geneva on Monday, amid outcry over the exclusion of Taiwan from participation.

China has blocked Taiwan's participation since 2017 as officials in Beijing consider the island democracy to be part of their territory.

Foreign Minister Joseph Wu and Health and Welfare Minister Chen Shih-chung expressed "deep displeasure" over the fact that Taipei was not invited to participate as an observer despite widespread global support from the G7 and global health organisations.

Wu and Chen urged the WHO to "serve the health and welfare of all humanity and not capitulate to the political interests of a certain member".

Australian Associated Press