Interior Secretary Eduardo Ano, who is helping to spearhead the country's virus response, said he returned a positive test on Saturday after experiencing flu-like symptoms last week.

He was first diagnosed with COVID-19 in March, but did not show any signs of the disease at the time.

People infected with coronavirus build up antibodies starting about a week after infection or the onset of symptoms, research has shown.

But scientists are still unsure whether the body systematically builds up enough immunity to ward off a new attack by the virus or, if it does, how long such immunity lasts.

Some studies have shown that patients who recover from coronavirus may lose their immunity within months, or even weeks. 

Health Undersecretary Maria Rosario Vergeire said experts were analyzing Ano's symptoms, his previous positive test, and laboratory results to see if this is a second infection. 

"Let's not call it a re-infection. The scientific community has not yet accepted that a re-infection occurs," Vergeire cautioned.

Ano was tested ahead of a meeting with President Rodrigo Duterte and other key cabinet members on Monday to decide whether to extend a two-week lockdown in Manila and four surrounding provinces that is due to expire this week.

The country's virus caseload has surged above 160,000 -- the highest in Southeast Asia -- with more than 2,600 deaths. More than a quarter of the infections remain active.

The health and labor ministries have ordered people to wear face masks and visors at their workplaces after outbreaks at office cafeterias.