Introduction

From March 24 to May 30, 2020, the Federal health authorities in Mexico recommended restricting operations in public institutions and private companies and staying at home to help contain the coronavirus disease 2019 (Covid-19).^1,2,3 Mobility decreased to 67%, yet, 75% of people still reported having to leave their house.^4,5 As in other low and middle-income countries, a large proportion of the Mexican population lives in multigenerational and crowded households with limited sanitary conditions and participate in informal and precarious jobs, forcing household members to work even under stay-at-home orders.⁶

Home confinement is a key non-pharmacological intervention for the Covid-19 response. Nevertheless, the effectiveness of this intervention may be reduced if household conditions increase the probability of transmission. Across the world, SARS-CoV-2 household transmission has been heterogeneous, ranging from 0% in South Korea to 45% in Treviso, Italy.⁷ Understanding the drivers of household transmission is key to increase the effectiveness and safety of home confinement.

Prior studies have shown that the number of SARS-CoV-2 seropositive members at home increases the more the family size.⁷ Although evidence is still limited, transmission seems to be higher in adults than in children and between spouses than among other family members.⁷ Less is known about the association between housing characteristics and seropositivity in households. However, potential proposed risk factors include a smaller number of bedrooms, poor sanitary services, and the use of air conditioning systems over natural ventilation.^8,9

We aimed to assess the potential association of family and housing characteristics with the number of seropositive cases to SARS-CoV-2 in Mexican households. Using data from the Encuesta Nacional de Salud y Nutrición (Ensanut) 2020 Covid-19, we selected households with at least two blood samples and at least one seropositive household member as a proxy of household transmission. Then, we compared family and household characteristics by the number of seropositive members and fitted adjusted models to identify potential conditions that could increase household transmission.

Materials and methods

Results

Among households with more than one member and at least one IgG anti-SARS-CoV-2 seropositive case, 49.6% had only one seropositive member, 36.2% had two, and 14.2% had three or more seropositive members (table I). Families composed of adults and older adults had a lower frequency of three and more seropositive members (4.7%), compared to households with children/adolescents and adults (15.6%), and to families with children/adolescents, adults, and older adults (18.7%). Households with more than two members per room had a higher prevalence of three and more seropositive members (18.3%), compared to households with two (12.5%) and less than two members per room (11.6%).

Table II⁵ presents the adjusted association between the number of seropositive household members and the familial composition and housing characteristics. Families composed of children/adolescents, adults, and older adults had, on average, 1.07 (95%CI: 1.03, 1.11) times as many members with anti-SARS-CoV-2 antibodies as families composed of adults and older adults. Households with more than two members per bedroom had, on average, 1.07 (95%CI: 1.05, 1.09) times the number of members with antibodies than households with less than two members per bedroom. The average number of seropositive members in households with natural ventilation was 6% lower than in households without it (95%CI: 0.92, 0.97).

Discussion

A higher number of seropositive members was observed in families with children/adolescents, adults, and older adults. The survey design did not allow us to identify the index case; however, it would be expected that fewer children and adolescents were in the index case because they have received virtual learning since March 2020, reducing their contact with non-household members. This might not be the case for adolescents who left their houses to conduct non-educational activities.¹⁷

A higher number of household members per bedroom was associated with a higher number of SARS-CoV-2 seropositive members in the household. Close interactions are expected to last longer if bedrooms are shared, facilitating disease transmission.¹² In Mexico, more than 50% of households have more than 3.7 inhabitants, and 9.7% of families live in overcrowded conditions, which could have facilitated infection.^18,19 We also observed a lower number of seropositive household members in houses with open windows, suggesting that natural ventilation could help reduce the probability of infection.

Our study has important limitations. We did not have complete coverage of household members or the specific infection dates, as required by household transmission studies. Therefore, we do not rule out that some individuals acquired the infection through other non-household members. Yet, we restricted comparisons to households with the same total number of members and, given the random selection of members, were able to examine the number of seropositive cases across households. Also, we cannot rule out the possibility of selection bias given the response rate. The sensitivity analysis by Abreu and colleagues suggest that individuals with Covid-19 related symptoms were more likely to provide a blood sample. However, the impact on the seroprevalence was small after accounting for this potential bias.^** While household transmission studies with complete coverage of household members will be needed to confirm our results, our analysis provides a first assessment of the potential role of familial composition and housing characteristics in SARS-CoV-2 infection.

Home confinement is a useful tool for disease transmission control. Yet, structural conditions in low- and middle-income countries could decrease its effectiveness. In our analysis, multigenerational and crowded households seemed to be at higher risk of infection. Our findings reiterate the importance of safe isolation at home in well-ventilated rooms, which may not be possible for an important proportion of the Mexican population. Given the structural limitations of families in our country, voluntary isolation centers should be made available, especially for multigenerational and crowded households.⁶