Cloth masks are better than nothing at protecting wearers and the people around them from disease, but they are not equivalent to medical-grade masks and shouldn’t be recommended for health care workers, researchers said Wednesday.

The researchers, from the University of New South Wales in Sydney, Australia, said their results showed cloth versions don’t work as well.

“In 2015, we conducted a randomized controlled trial to compare the efficacy of cloth masks with that of medical masks and controls (standard practice) among healthcare workers in Vietnam,” Dr. Abrar Chughtai, an epidemiologist, and colleagues wrote in the journal Emerging Infectious Diseases. They checked the workers for influenza-like illnesses, which include a range of respiratory infections.

“Rates of infection were consistently higher among those in the cloth mask group than in the medical mask and control groups. This finding suggests that risk for infection was higher for those wearing cloth masks,” they added.“The mask tested was a locally manufactured, double-layered cotton mask. Participants were given five cloth masks for a 4-week study period and were asked to wash the masks daily with soap and water. The poor performance may have been because the masks were not washed frequently enough or because they became moist and contaminated.”

The team also reviewed 19 other studies of masks and found the fabric used is important.

“Filtration effectiveness of cloth masks depends on many factors, such as thread count, number of layers, type of fabric, and water resistance,” they wrote. “Current evidence suggests that multilayered masks with water-resistant fabric, high number of threads, and finer weave may be more protective.”

The researchers said that for coronavirus, all frontline health care workers should wear medical masks or N95 respirators.

“During a pandemic, cloth masks may be the only option available; however, they should be used as a last resort when medical masks and respirators are not available,” they concluded.