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Changes in contact patterns shape the dynamics of the COVID-19 outbreak in China

Our take —

As a result of social distancing measures, total interpersonal contacts dropped significantly in China, and mixing among school-aged children was almost entirely eliminated. The study estimates that school closures by themselves would reduce peak incidence by a considerable amount, though they would not be sufficient to completely halt transmission. In contrast to a previous study, significant differences in susceptibility to infection by age were observed (younger individuals appeared to have lower susceptibility), though these results come from a sample of index cases that may not represent the age structure and symptom profile of all SARS-CoV-2 infections.

Preprint Review

This expert summary is for the peer-reviewed article linked above. We also summarized this paper before it underwent peer-review.  You can find the original review of the preprint by clicking here.

Study design

Cross-sectional; Ecological; Modeling/Simulation

Study population and setting

This study examined contacts reported by 636 participants in Wuhan, China, and 557 participants in Shanghai, China. The surveys were conducted from February 1 to 10, 2020, and participants were asked to report contacts on two days: a typical weekday in the pre-epidemic period of December 24 to 30, 2019, and then on the day before the survey was conducted. A contact was defined as either direct physical contact or a two-way, in-person conversation of three or more words. The authors used the survey data to estimate changes in age-mixing contact patterns. In a separate analysis of age-specific infection susceptibility, the authors used data from the Hunan CDC field contact tracing and testing of 136 index cases in Hunan Province, China and 7,375 of their contacts, identified between January 15 and March 1, 2020. Close contacts of confirmed cases were involuntarily isolated and tested for SARS-CoV-2; authors estimated odds ratios of infection for different age strata. Finally, the authors used estimated contact patterns and age-specific susceptibility in an SIR model to 1) estimate how differences in age-mixing patterns have affected SARS-CoV-2 transmission, and 2) how school closures and social distancing would be likely to affect future transmission.

Summary of Main Findings

Daily contacts dropped from 14.6 to 2.0 in Wuhan and from 18.8 to 2.3 in Shanghai. The highest density of contacts in the baseline period was observed among school-aged children; in the outbreak period, this mixing almost entirely disappeared, leaving only within-household mixing. Estimated susceptibility to SARS-CoV-2 infection increased with age: relative to the reference category of ages 15-64, the odds ratio (OR) of infection for children under 15 was 0.34 (95% CI: 0.24, 0.49); the OR for adults 65 and older was 1.47 (95% CI: 1.12, 1.92). Alterations in mixing patterns were estimated to decrease the reproduction number (R0) considerably. Social distancing measures (the broad aggregate of all measures implemented in China) were sufficient by themselves to extinguish the epidemic. Simulating pre-emptive school closures without any other intervention reduced peak incidence by 42-64%, depending on assumptions about the degree to which school-aged contacts were diminished. School closures were not sufficient to extinguish the epidemic by themselves, however.

Study Strengths

Age-specific contact matrices were estimated from surveys, and age-specific susceptibility to infection was estimated from detailed contact tracing data.

Limitations

Responses to contact surveys may be limited by recall bias, particularly for the pre-epidemic period. Respondents may have under-reported contacts in the epidemic period due to social pressure. If the age profile of the index cases and their transmission chains is not representative of the population of all individuals infected with SARS-CoV-2 (e.g., because younger individuals may experience less severe symptoms or are more commonly asymptomatic), age-specific susceptibility estimates may be biased. Estimates are derived from a small number (136) of index cases and their contacts in one geographic area. As with most studies of NPIs in China, it is difficult to isolate the effects of individual interventions since multiple measures were implemented nearly simultaneously.

Value added

This is one of the few studies to offer estimates of age-specific susceptibility to SARS-CoV-2 infection, and to estimate effects of school closures on the reproductive number.