The risk of transmitting the coronavirus to members of the same household is large but not as large as you probably thought. Simple protection measures can go a long way, the authors of a new study conclude.

Although we’ve learned so much about SARS-CoV-2, the virus behind the pandemic, we still don’t know exactly how it’s transmitted. We know it’s passed through droplets, but there is also evidence that airborne transmission is possible; we know surface transmission is less likely, but we still don’t know if it plays a significant part; we also know that indoor transmission is more likely than outdoor, but again, just how likely remains very uncertain.

These questions aren’t easy to answer. Even for the flu, which has been studied for decades, the details of transmissibility remain difficult to crack, which is why observational studies are so important. Basically, since it’s so unclear how exactly a virus spreads, we can get an idea by studying the effects of transmission in a population. The more controlled the environment, the more accurate the assessment. This is exactly what a team led by Zachary J. Madewell at the Department of Biostatistics, University of Florida has done.

Madewell and colleagues wanted to analyze how likely it is for COVID-19 to be transmitted from one household member to another. They started with a database of 485 reports and screened them, selecting only the ones that offered a comprehensive assessment. Ultimately, they ended up with 40 studies that also included contact tracing, whose results were compared and analyzed. The resulting data came from multiple countries in Europe, Asia, Australia, and North America.

The rate of infection between members of the same household (called the secondary attack rate or SAR) varied substantially between different studies, but it was never above 50%. In fact, the highest observed SAR was just over 44%, and in the vast majority of studies, the rate was under 33%.

The household is a favorable environment for transmission due to the frequency of contacts between family members, reduced usage of personal protective equipment, shared living and eating environment, and persistence of SARS-CoV-2 on different surfaces. In light of all of this, it’s remarkable that over all the analyzed studies, the average SAR was only 18.8%.

The researchers also compared how different aspects of the household increased or decreased the transmission rate. Unsurprisingly, the more contact infected people had with household members, the likelier it was to transmit the virus — but other than that, there were few clear conclusions.

“We observed that household SARs were significantly higher from symptomatic index cases than asymptomatic index cases, to adult contacts than children contacts, to spouses than other family contacts, and in households with one contact than households with three or more contacts,” the researchers note.

Overall, it’s still a high infection rate and researchers warn that it should be addressed (with strategies such as mask-wearing at home, improved ventilation, and voluntary isolation).

The study was published in medrXiv.