A senior World Health Organization official yesterday urged national authorities to make a priority of understanding the long-term consequences of coronavirus infections as some people show worrying symptoms months later.

"The burden is real and it is significant. About one in 10 Covid-19 sufferers remain unwell after 12 weeks, and many for much longer," Hans Kluge, regional director for WHO Europe, told a press conference.

"It's a clear priority for WHO, and of the utmost importance. It should be for every health authority".

While some studies are beginning to shed light on the illness, it is still unclear why some patients with Covid-19 continue to show symptoms for months, including tiredness, brain fog, and cardiac and neurological disorders.

Kluge stressed that those patients "need to be heard if we are to understand the long-term consequences and recovery from Covid-19."

WHO Europe called on European countries and institutions to "come together as part of an integrated research agenda," harmonising data collection tools and study protocols.

In boost for Covid-19 battle, the Pfizer vaccine has proven 94 percent effective in a study involving 1.2 million people in Israel, the first peer-reviewed real world research confirming the power of mass immunization campaigns to bring the pandemic to a close.

Moderna Inc said on Wednesday it is working with US government scientists to study an experimental booster shot that targets a concerning new variant of the coronavirus. The US biotech company has produced raw material for a booster shot aimed at addressing the virus variant first found in South Africa that may be more resistant to existing vaccines, it said.