Symptomatic patients four times more likely to spread virus: Study

A person who has symptoms of Covid-19 is four times more likely to pass the virus on to others than someone who remained asymptomatic, and the highest risk of infection comes from sharing a home with an infected individual, according to a statistical analysis of dozens of contact-tracing reports, which offers new evidence to underscore the need for people to isolate themselves as soon as they develop signs they may be ill.

The researchers from Imperial College London also found that households have the highest transmission rates among indoor settings when compared to being in a workplace and casual social spaces, with the chance of one family member infecting another becoming significantly higher if the duration of exposure is more than 5 days.

“This analysis provides some of the first evidence that asymptomatic infections are substantially less infectious than symptomatic cases. It also reinforces growing evidence of the importance of household transmission, especially in the context where symptomatic cases are not isolated outside the home,” said Imperial College professor Neil Ferguson, whose modelling in March led the UK to announce a lockdown to stop transmission, in comments over email.

The study was based on a meta-analysis – a uniform statistical review – from 45 contact-tracing studies from around the world published till mid-July. “Where the initial case was asymptomatic, the secondary attack rate was estimated to be two thirds lower (3.5%, 95% CI: 0.0%-6.4%) than when the index case had symptoms (12.8%, 95% CI: 8.9%-16.7%, p = <0.001),” Imperial College’s MRC Centre for Global Infectious Disease Analysis said in a statement.

Asymptomatic transmission has been seen as a particular challenge in stopping the spread of Covid-19 since contact-tracing efforts often fail to identify the index patient, who may not have had any signs that they were infected when they passed the virus on.

But outbreaks have also perplexed epidemiologists when they noticed that only a small proportion of infected people cause the most number of infections – a so-called superspreader effect.

“The differences we observed in transmissibility by symptom status of index cases and duration of exposure have important implications for outbreak control strategies, highlighting how contact tracing, testing and rapid isolation of cases will be crucial,” said lead author Hayley Thompson.

The authors also calculated chances of infections in different settings. Among workplace contacts, they found three studies showed there was a 1.9% chance of a secondary infection in an office.

In healthcare-based contacts -- exposures in hospitals, clinics etc -- the researchers found a secondary attack rate of 3.6% of all contacts of the average index patients.

In social settings, which included people exposed while travelling, at religious events, fitness classes, whilst shopping or at entertainment venues and other social events with family and friends, the pooled attack rate came to about 1.2%.

The risk, thus, was highest among household contacts, and the chances increased with time. “Chance of passing on infection in households (the secondary attack rate) estimated to be 21.1% (95% CI: 17.4%-24.8%),” the report added.

The researchers found that there were no statistically significant differences in how people of different ages spread the infection.

These findings are crucial in order to fix strategies to test, trace and isolate people. “Understanding the conditions where transmission is more likely to occur is essential in guiding policy interventions to reduce transmission whilst balancing the economic impact,” said Imperial College researcher Andria Mousa.

“Our results highlight the importance of targeting interventions in settings where close and prolonged contacts occur, as well as the need for further research to inform policies for reducing transmission in workplaces, schools and care-homes,” Mousa added.

Thompson, however, added that more studies are required to better understand these patterns. “The number of studies we identified from large scale contract tracing was limited and highlights the need for continued research into further understanding the locations and types of contact that facilitate transmission, especially in schools and workplaces.”