Microsoft co-founder and philanthropist Bill Gates announced that he has tested positive for COVID-19. Gates, who has donated millions to pandemic relief efforts, announced on Tuesday via Twitter.

Mr Gates, 66, said on Twitter that he was isolated with mild symptoms and that he was vaccinated and boosted. It was not clear if this was the first time he had tested positive for the coronavirus.

In a tweet, Gates said the Bill & Melinda Gates Foundation planned to meet Tuesday for the first time in two years. Gates says he will join via Microsoft Teams as he thanks the foundation for its hard work.

In recent months, Mr Gates, one of the richest men in the world, has focused his considerable resources on the pandemic. He published a book last week called “How to Prevent the Next Pandemic,” in which he explains how countries could pull off a coordinated effort to avert pandemics and “eliminate all respiratory diseases.”

He has been outspoken about how the global health authorities should respond to the pandemic and distribute vaccines. The Bill & Melinda Gates Foundation said it has donated millions to organizations including Gavi and the World Health Organization to fund testing, treatments and vaccine distribution. (The Gateses divorced last year, but both expressed continued commitment to the foundation.)

Bill Gates has been a vocal proponent of pandemic mitigation measures, especially access to vaccines and medication for poorer countries. The Gates Foundation in October said it will spend $120 million to boost access to generic versions of drugmaker Merck’s antiviral COVID-19 pill for lower-income countries.

In January, Mr Gates said on Twitter that “once Omicron goes through a country,” the virus could be treated “more like seasonal flu.”