The head of the World Health Organization (WHO) has said Covid-19 is easily the most severe global health emergency it has ever declared.

Dr Tedros Adhanom Ghebreyesus said he would reconvene the WHO’s emergency committee this week to review its assessment of the pandemic.

There have been five other global health emergencies: Ebola (two outbreaks), Zika, polio and swine flu.

More than 16m cases of Covid-19 have been reported since January.

“When I declared a public health emergency of international concern on 30 January… there were less than 100 cases outside of China, and no deaths,” Dr Tedros said.

“Covid-19 has changed our world. It has brought people, communities and nations together, and driven them apart.”

• Spain races to save tourism as cases surge
• UK in talks over air bridges for Balearic and Canary Islands

“This is the sixth time a global health emergency has been declared under the international health regulations, but it’s easily the most severe. [More than] 16m cases have now been reported to WHO and more than 640,000 deaths, and the pandemic continues to accelerate.

“In the past six weeks, the total number of cases has roughly doubled,” he added.

In other developments:

• US President Donald Trump’s national security adviser, Robert O’Brien, has tested positive for coronavirus. He is the highest-ranking official in the administration known to have tested positive
• Spain insists recent outbreaks of new cases were isolated and the country is safe for tourists to visit, after the UK introduced new measures requiring visitors from Spain to quarantine for 14 days
• Vietnam has closed the coastal city of Da Nang to tourists, after 15 new locally transmitted coronavirus cases were recorded there – the first in the country since April
• The US has signed a contract with pharmaceutical company Pfizer to buy 100 million doses of its Covid-19 vaccine once it is approved for use
• Belgium is tightening restrictions to try to avoid another lockdown because of a worrying rise in the number of its cases. From Wednesday, Belgians will be allowed to see a maximum of five people outside of their families. Currently a Belgian individual can meet 15 people in a “social bubble”

At Monday’s briefing in Geneva, Switzerland, the WHO also said travel restrictions could not be the answer for the long term, and countries had to do more to halt the spread by adopting proven strategies such as social distancing and wearing masks.

“It is going to be almost impossible for individual countries to keep their borders shut for the foreseeable future. Economies have to open up, people have to work, trade has to resume,” WHO emergencies programme director Mike Ryan said.