Researchers at Imperial College London found that household surfaces touched by an infected person can act as a vector of transmission.
The study of 279 households in London was conducted at the height of the pandemic during the alpha and pre-alpha waves of infection.
It involved 414 susceptible contacts living in the same households as 279 newly diagnosed cases between August 1, 2020 and March 31, 2021. All participants were aged between 6 and 79 years.
As the study was conducted early in the pandemic, very few of the participants had been vaccinated or previously infected and were therefore more susceptible to infection.
All contacts were given a regular PCR test for Covid infection, while researchers also took swabs from the hands of infected participants as well as the most frequently touched surfaces in communal areas – such as a fridge door or kettle handles.
The researchers looked for correlations between microbiological detection of the virus on hands and surfaces and transmission to household contacts.
They found that if the virus was detected on an infected person’s hands, then contacts in their household were 1.7 times more likely to get infected than those in households where the infected did not have the virus on their hands.
Similarly, the presence of virus on an infected person’s hands was associated with a three times greater risk of contacts in the household having a positive hand-swab.
As a result, contacts with the virus on their hands were twice as likely to become infected with Covid-19.
If virus was present on frequently touched surfaces in the household, contacts were 3.8 times more likely to have detectable virus on their hands and 1.7 times more likely to be infected.
The findings support the use of interventions at home when someone has a Covid infection, in particular frequent handwashing and regular surface disinfection.
Professor Ajit Lalvani, lead author of the study, and Director of NIHR HPRU in Respiratory Infections, said: “There’s no doubt that if you have Covid-19 you’re emitting the virus into the air as micro-aerosols as well as large droplets that land on your hands and the surfaces around you. What hasn’t been shown, until now, is that the presence of the virus on people’s hands or household surfaces predicts transmission to contacts.
“Our real-life study in London households provides the first empirical evidence to show that the presence of SARS-CoV-2 on people’s hands and surfaces contributes significantly to spread of Covid-19. Since we didn’t systematically sample household air, we cannot rule out airborne transmission occurring in parallel.”