Current evidence suggests the treatment does not improve survival or reduce the need for ventilation in COVID-19 patients, the World Health Organization says.

The decision adds to evidence that the benefits of plasma infusions from COVID-19 survivors may be limited, but advocates say the case isn’t closed.

A study of people in Sierra Leone suggests that the virus can lie in hiding from the immune system before re-emerging later and sparking a new response—although researchers didn’t examine whether this could make people infectious again.

A randomized controlled trial on the use of convalescent plasma therapy to treat coronavirus infections—the first in the world to be completed—yields disappointing results, but some doctors are not discouraged.

Researchers say the popularity of convalescent plasma makes it more difficult to gather high-quality data on its efficacy.

Under emergency protocols, doctors can request to use survivors’ plasma to treat some critically ill COVID-19 patients.