face masks at home RESTRICTED
CNN  — 

One of the few reliefs in our current pandemic is removing that mask when you arrive back home after a trip to the store. If you’ve got family there, however, a new study suggests you may want to keep it on.

The study, which was published in BMJ Global Health on Thursday, showed that wearing a mask at home was 79% effective at preventing the spread of the virus – but only when family members started wearing masks before symptoms emerged in the first person infected. Cleaning the house frequently with bleach or disinfectants was almost equally effective at 77%.

“This study confirms the highest risk of household transmission being prior to symptom onset, but that precautionary [non-pharmaceutical interventions], such as mask use, disinfection and social distancing in households can prevent Covid-19 transmission during the pandemic,” the study says.

In February, Chinese officials said most of the cluster cases they studied weren’t coming from supermarkets or schools, but rather within families. Of the more than 1,000 cluster cases studied, 83% were identified as family clusters, according to Wu Zunyou, a virus expert with China’s Center for Disease Control and Prevention.

While it was known that social distancing and mask wearing could prevent transmission of the disease in the community, there was little evidence to prove whether it was effective within families.

For the new study, researchers called 460 people from 124 families in Beijing who were living with an infected person and questioned them on their household hygiene and other behaviors during the pandemic.

Researchers found 41 out of the 124 families saw the virus being transmitted from the first infected person to other family members, totaling to 77 adults and children being infected. But the families who cleaned their homes with disinfectants daily, opened their windows and kept at least 1 meter (3 feet) apart were at lower risk of passing the virus, even in more crowded households.

Families who engaged in close daily contact, such as eating meals around a table or watching TV together were associated with an 18-fold increased risk. Daily close contact with a family member who was showing symptoms increased the risk for others, even if they started wearing masks at that point, according to the study.

The study is not without its critics

Some members of the scientific community who didn’t participate in the study are recognizing its significance.

“This is an important paper because it comes at a time when – as lockdown is eased – the risk of a person entering the home who has become infected (e.g. whilst on public transport or in the workplace) but is unaware that this is so, is increasing,” Professor Sally Bloomfield of London School of Hygiene and Tropical Medicine said in a statement.

Professor Trish Greenhalgh of University of Oxford mirrored her response, saying that the practice of wearing masks at home is “perhaps the most interesting as it’s something that few people currently do in their own homes, especially when not symptomatic.”

“Worryingly, whereas people who wore a mask before they became symptomatic with Covid-19 were significantly less likely to pass it on to others in the household, those who only started wearing the mask after they became symptomatic weren’t able to protect their family members,” Greenhalgh added.

But others have pointed to the study’s limitations, including ones that the authors also recognized.

“Telephone interview has inherent limitations including recall bias,” the study says. “The evaluation results of mask wearing were reliable, but we did not collect data on the concentration of disinfectant used by families.”

Dr. Antonio Lazzarino at the University College London said that the study itself is not enough to make any official recommendations of any kind.

“This study is not robust science, as it has several limitations in the conception and in the statistical analysis,” he said in a statement. “The main limitation is that it was designed at the family level, rather than at the individual level.”

Lazzarino also pointed out that the study didn’t make a distinction between whether the families were wearing N95 masks, surgical masks or basic cloth masks.