Two new studies explore the risk of SARS-CoV-2 transmission among household members, one finding that children and adults are at similar risk and one showing that COVID-19–naïve family members' risk was 45% to 97% lower, depending on the number of members immune through infection or full vaccination.
Transmission comparable across age groups
Late last week, a team led by Centers for Disease Control and Prevention researchers published interim data from an ongoing study in JAMA Pediatrics involving a convenience sample of 1,236 children and adults in 310 Utah and New York City households with at least one child. Participants submitted self-collected nasal swabs for COVID-19 testing and completed symptom questionnaires each week from September 2020 through April 2021.
Average risk of SARS-CoV-2 infection in 40 households with at least one COVID-19 case was 52% (range, 11% to 100%). Risk was higher in New York City than in Utah, at 80% versus 44% (95% confidence intervals [CIs], 64% to 91% and 36% to 53%, respectively).
SARS-CoV-2 infection incidence rates were 3.8 (95% CI, 2.4 to 5.9) and 7.7 (95% CI, 4.1 to 14.5) per 1,000 person-weeks in Utah and New York City, respectively, for an age-adjusted rate of 5.8 (95% CI, 3.1 to 8.4).
Site-adjusted incidence rates per 1,000 person-weeks were comparable across age groups, at 6.3 per 1,000 person-weeks for children 0 to 4 years (95% CI, 3.6 to 11.0), 4.4 for those 5 to 11 (95% CI, 2.5 to 7.5), 6.0 for those 12 to 17 (95% CI, 3.0 to 11.7), and 5.1 for adults (95% CI, 3.3 to 7.8).
Thirty-six percent of all participants submitted a respiratory sample after one or more episodes of symptomatic illness, and 8% tested positive for COVID-19. No COVID-19 cases were identified outside of the study or among vaccinated participants.
Asymptomatic illness in up to 52% of kids, 12% of adults
The proportions of asymptomatic COVID-19 were 52% for children 0 to 4 years, 50% for those 5 to 11, 45% for those 12 to 17, and 12% among adults. "A larger fraction of SARS-CoV-2 infections in children were asymptomatic and would likely have gone undetected without study testing, supporting hypotheses that SARS-CoV-2 infections among children have been substantially underascertained during the COVID-19 pandemic," the authors wrote.
Households had a median of four members and two adults, and almost half of participants were 18 and older (47%) while 14% were 0 to 4 years old, 25% were 5 to 11, and 13% were 12 to 17. More than 47% of households reported annual incomes of $100,000 or more, while 7% had incomes below the poverty line.
Twenty-three percent of participants were Hispanic, and 71% were White. Of adults, 57% received one or two doses of a COVID-19 vaccine over the study period (19% partially and 38% fully vaccinated). Of the 652 children 0 to 17 years, 71% attended daycare or school at some point during the study.
The researchers noted that the risk of SARS-CoV-2 infection in adults and children will change amid increasing COVID-19 vaccine uptake and circulation of virus variants of concern.
"Our findings suggest that SARS-CoV-2 infection prevention strategies, such as handwashing, masking, physical distancing, and COVID-19 vaccination should target children in addition to adults to both mitigate individual health outcomes for children and reduce the overall burden of SARS-CoV-2 infection in the community," they concluded.
Flor Munoz, MD, of Baylor College of Medicine, said in a linked commentary that the study findings can inform school, daycare, extracurricular activity, and camp planning, as well as public health strategies.
"Pediatric populations, from infants to adolescents, must be included in pandemic preparedness, disease burden assessment, timely vaccine and therapeutics research, and strategies and policies for the control of outbreaks and pandemics from day 1 without delay," she wrote.
Vaccination slows spread in households
A study led by Umea University in Sweden, published today in JAMA Internal Medicine, studied 1,789,728 members of 814,806 Swedish families with at least one family member who had either had a previous SARS-CoV-2 infection or was fully vaccinated against COVID-19 by Apr 14. Each immune person was matched in a 1:1 ratio to a nonimmune member of a family with two to five members. Average follow-up time was 26.3 days.
By May 26, 5.7% of 1,549,989 nonimmune family members tested positive for COVID-19. Families with one immune member had a 45% to 61% lower chance of infection than those with no immune member (hazard ratio [HR], 0.39 to 0.55).
The risk reduction rose to 75% to 86% in families with two immune members (HR, 0.14 to 0.25), 91% to 94% with three immune members (HR, 0.06 to 0.09), and to 97% with four immune members (HR, 0.03). The results were comparable for the outcome of severe COVID-19 leading to hospital admission. For instance, in a three-member family with two immune members, the nonimmune member had an 80% lower risk of infection (HR, 0.20).
"The results strongly suggest that vaccination is important not only for individual protection, but also for reducing transmission, especially within families, which is a high-risk environment for transmission," lead author Peter Nordstrom, MD, said in an Umea University news release.
The results were similar, regardless of whether the member became immune through previous infection or one or two vaccine doses. The authors, however, noted that the study was conducted when the Alpha (B117) SARS-CoV-2 variant was dominant in Sweden, so the results may not apply to the more transmissible Beta (B1351) or Delta (B1617.2) variants that have emerged since then.
"For instance, a single dose of the BNT162b2 [Pfizer/BioNTech] and ChAdOx1 nCoV-19 [AstraZeneca/Oxford] vaccines has been associated with a modest protection (approximately 30%) against the Delta variant, which seems to be the dominating variant in the most recent wave of the pandemic," the researchers wrote. "Although the results of this study indicate similar benefits of immunity from a single vaccine dose and full vaccination, the evidence on emerging variants may promote full vaccination."