Risk of COVID-19 transmission in Singapore households increases with age

Older adults and elderly individuals are still more susceptible to acquiring COVID-19 infection than younger individuals, even if individuals from both age groups stayed in the same household, a Singapore study has shown.

“Determining the transmission of SARS-CoV-2 in households is crucial for understanding the heterogeneities in the age-related risk of infection as the exposures will likely be high given the strong familial interactions,” said the researchers.

Prior to the implementation of distancing measures in Singapore, the team monitored close family contacts (n=875 from 265 households) of the first 400 COVID-19 cases identified before March 21, 2020 to determine the risk of SARS-CoV-2 infection with age. Among the COVID-19 cases, 277 were primary or coprimary cases in their household, 43 tested positive for SARS-CoV-2, while 34 had no household contacts. The remaining 46 cases and their households were excluded, either due to common exposure between cases and household contacts, or being advised to self-isolate from the rest of their household. Overall median household size was four. [Influenza Other Respi Viruses 2021;15:206-208]

The mean probability of infection increased with age across all durations of exposure – from 5 days (2, 5, and 10 percent for ages 10, 50, and 80 years, respectively) up until 20 days (6, 14, and 25 percent, respectively).

Further adjustments by gender and household size showed a similar trend at both the 5- and 20-day mark among individuals aged 10 (3 percent and 9 percent, respectively), 50 (7 percent and 19 percent) and 80 years (12 percent and 31 percent).

The findings reinforce observations from local preschool outbreaks showing adult staff acquiring COVID-19 infection as opposed to none among the preschool students despite being in close contact with infected adults. “These data suggest that children are less likely to be infected. [Hence, they are] not the major drivers of transmission unlike in other respiratory diseases such as influenza and [respiratory syncytial virus],” the researchers pointed out. [Proc Natl Acad Sci USA 2011;108:2825-2830]

While the results imply that school closures do not appear to be the primary community measure that must be taken to reduce disease transmission, it was still implemented as part of social distancing protocols. “This could serve to minimize interactions between adult and elderly workers in schools and to ensure that parents remain home to care for their children,” they said.

However, the time of evaluation may have influenced the findings, as this was conducted prior to the implementation of tightened health safety measures. “[This] may have modified the risk of acquiring infection [at] home,” noted the researchers.

Also, as healthy household contacts were not tested (only symptomatic contacts and asymptomatic children ≤12 years were referred for testing), the true proportion of asymptomatic cases and secondary infection rates were not determined. The zero index cases in children also limited the chance to assess infectiousness of COVID-19 by age bracket.

“[Nonetheless, our findings are] important to guide containment measures for the ongoing pandemic such as school closures and healthcare resource allocation,” said the researchers.