An analysis during the SARS-CoV-2 Omicron variant wave in Japan suggests a lower rate of in-home COVID-19 transmission when the index (primary) patient or contact were fully vaccinated against COVID—and no transmission when both parties were fully vaccinated.
The study was published late last week in the International Journal of Infectious Diseases.
Higher Omicron secondary attack risk
For the Vaccine Effectiveness, Networking, and University Safety (VENUS) study, Kyushu University researchers analyzed data on COVID-19 vaccination and infections and a resident registry in two municipalities in Japan and modeled the effect of vaccination on transmission.
The evaluation included 7,326 households with a member infected by SARS-CoV-2 from January to April 2022 and 17,586 contacts. Households were categorized by whether they included children 11 years and younger, with 2,702 having children in that age-group (household group 1) and 4,624 with older children (household group 2).
Full vaccination was considered three COVID-19 vaccine doses, with the third dose given at least 7 days earlier. All others were classified as unvaccinated, including recipients of one dose (0.9%), two doses (43.9%), and three doses, with the last dose given less than 7 days earlier (22.2%).
"The Omicron variant and its subvariants have demonstrated a higher household secondary attack risk (SAR) than the previously dominant B.1.617.2 (Delta) variant, which indicates the need for more effective measures to reduce the formation of household clusters," the authors wrote.
No infections when both parties vaccinated
The percentage of household contacts infected via an index patient was 24.3% in all households, 35.0% in household group 1 and 15.4% in household group 2. Of all households, participants with older children accounted for 74.4% of index patients.
In household group 1, younger children made up 56.3% of index patients. Among all households, symptom onset in secondary patients occurred within 2 or 3 days of the index case.
This study demonstrated the effectiveness of COVID-19 vaccination as a strategy to prevent secondary transmission within households.
The odds ratio (OR) for household spread of COVID-19 was less than 0.6 when the index patient and/or contact were vaccinated. Among households with children 11 years or younger, the OR was 0.71 when only the contact was vaccinated. The odds of household transmission were 45% higher (OR, 1.45) when the index case was 11 years or younger.
No infections were detected when both the index patient and contact were vaccinated.
Almost 70% higher infection rate in women
The SAR was estimated at 24.3% in all households, 35.0% in household group 1 and 15.4% in household group 2. The chance of household spread was about 70% higher when the contact was a woman aged 20 to 59 years than a man of the same age (OR, 1.68 or higher) in all households.
"This could indicate that young children as primary cases require an unavoidable need for care that is frequently fulfilled by mothers (represented by women aged 20–59 years in this study), thereby increasing their contact and risk of transmission," the researchers wrote.
When the household contact was 11 years or younger, the probability of household spread was roughly 45% to 47% higher than among men 20 to 59 years old. When a household had children aged 11 and younger, the likelihood of transmission was 153% higher than in households without children in that age-group.
"This study demonstrated the effectiveness of COVID-19 vaccination as a strategy to prevent secondary transmission within households," the researchers wrote. "Notably, the OR was high when the primary case was a young child aged ≤11 years, suggesting that further studies are needed to investigate the infectiousness and susceptibility of children according to vaccination status."
