The agency delayed endorsing masks for the general public until Friday, claiming there was too little evidence that they prevented transmission of the virus. Virtually all scientists and governments have been recommending masks for months.

The W.H.O. has said repeatedly that small airborne droplets, or aerosols, are not a significant factor in the pandemic’s spread, although a growing body of evidence suggests that they may be.

“The W.H.O. has been out of step with most of the world on the issue of droplets and aerosols,” said Michael Osterholm, an infectious disease expert at the University of Minnesota.

These scientific disagreements have wide policy implications. Many countries, including the United States, adopted lockdown strategies because they recognized that isolating only people who were sick might not be enough to contain the epidemic.