Household transmission of SARS-CoV-2: a systematic review and meta-analysis of secondary attack rate
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Abstract
Background
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is spread by direct, indirect, or close contact with infected people via infected respiratory droplets or saliva. Crowded indoor environments with sustained close contact and conversations are a particularly high-risk setting.
Methods
We performed a meta-analysis through July 29, 2020 of SARS-CoV-2 household secondary attack rate (SAR), disaggregating by several covariates (contact type, symptom status, adult/child contacts, contact sex, relationship to index case, index case sex, number of contacts in household, coronavirus).
Findings
We identified 40 relevant published studies that report household secondary transmission. The estimated overall household SAR was 18·8% (95% confidence interval [CI]: 15·4%–22·2%), which is higher than previously observed SARs for SARS-CoV and MERS-CoV. We observed that household SARs were significantly higher from symptomatic index cases than asymptomatic index cases, to adult contacts than children contacts, to spouses than other family contacts, and in households with one contact than households with three or more contacts.
Interpretation
To prevent the spread of SARS-CoV-2, people are being asked to stay at home worldwide. With suspected or confirmed infections referred to isolate at home, household transmission will continue to be a significant source of transmission.
Article activity feed
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SciScore for 10.1101/2020.07.29.20164590: (What is this?)
Please note, not all rigor criteria are appropriate for all manuscripts.
Table 1: Rigor
Institutional Review Board Statement not detected. Randomization not detected. Blinding not detected. Power Analysis not detected. Sex as a biological variable not detected. Table 2: Resources
No key resources detected.
Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).
Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:Our study had several limitations. The large amount of unexplained heterogeneity across studies could be attributable to variability in study definitions of index cases and household contacts, frequency and type of testing, …
SciScore for 10.1101/2020.07.29.20164590: (What is this?)
Please note, not all rigor criteria are appropriate for all manuscripts.
Table 1: Rigor
Institutional Review Board Statement not detected. Randomization not detected. Blinding not detected. Power Analysis not detected. Sex as a biological variable not detected. Table 2: Resources
No key resources detected.
Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).
Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:Our study had several limitations. The large amount of unexplained heterogeneity across studies could be attributable to variability in study definitions of index cases and household contacts, frequency and type of testing, sociodemographic factors, household characteristics (e.g., density, air ventilation), rates of community transmission, and local policies (e.g., centralized isolation). Many of the studies involved testing symptomatic household contacts, which likely missed asymptomatic infections, although SAR estimates were similar across studies testing all contacts and only symptomatic contacts. Conversely, the estimates may overestimate household transmission from index cases to contacts because studies cannot typically rule out infection from outside the home (e.g., non-household contact, fomite). Important questions remain about the household spread of SARS-CoV-2 including the efficiency of asymptomatic transmission, probability of fecal-oral transmission, role of children in potential of reinfection, and sexual transmission of SARS-CoV-2.102 To prevent the spread of SARS-CoV-2, people are being asked to stay at home worldwide. With suspected cases frequently referred to isolate at home, household transmission will continue to be a significant source of transmission. Prevention strategies such as increased mask-wearing in the home, improved ventilation, voluntary isolation at external facilities, and targeted antiviral prophylaxis should be explored.
Results from TrialIdentifier: No clinical trial numbers were referenced.
Results from Barzooka: We did not find any issues relating to the usage of bar graphs.
Results from JetFighter: We did not find any issues relating to colormaps.
Results from rtransparent:- Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
- Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
- No protocol registration statement was detected.
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Our take
This meta-analysis used 40 prior studies to estimate the rate of household secondary transmission for SARS-CoV-2. It estimated a secondary attack rate of 19.0% in households and 18.1% among family contacts. Overall, 32.5% of households with an index case had secondary transmission and spouses and adults were at increased risk compared to other relatives. The study had notable limitations based on the studies it drew from, and differences between how those prior studies identified and tested potential cases, their community transmission rate, and the potential lack of testing asymptomatic cases may result in misestimation in the meta-analysis. Still, this review represents one of the largest analyses of household secondary transmission, and provides important estimates of risk as different communities reopen.
Stud…
Our take
This meta-analysis used 40 prior studies to estimate the rate of household secondary transmission for SARS-CoV-2. It estimated a secondary attack rate of 19.0% in households and 18.1% among family contacts. Overall, 32.5% of households with an index case had secondary transmission and spouses and adults were at increased risk compared to other relatives. The study had notable limitations based on the studies it drew from, and differences between how those prior studies identified and tested potential cases, their community transmission rate, and the potential lack of testing asymptomatic cases may result in misestimation in the meta-analysis. Still, this review represents one of the largest analyses of household secondary transmission, and provides important estimates of risk as different communities reopen.
Study design
other
Study population and setting
This systematic review and meta-analysis described the rate of further infections (secondary attack rate, SAR) within households where at least one person tested positive for SARS-CoV-2. It identified 485 published articles through database searches; screening procedures removed duplicates and examined relevance, whether full-text versions were available, and whether they reported the rate of secondary transmission in households. After screening and assessment, 40 published studies were included in the meta-analysis.
Summary of main findings
There was substantial heterogeneity across the 40 studies. Overall, there was an estimated mean secondary attack rate of 19.0% (95% CI: 14.9 – 23.1%) in a household, and 18.1% (95% CI: 12.9 – 34.8%) among family contacts. There was no statistically significant difference between household and family secondary infection rates, however having a reported close contact with someone infected did increase the SAR by 4.3% (95% CI: 2.9 – 5.6%). There was a statistically significantly (P<0.001) higher SAR from people who were symptomatic and brought the infection home to household contacts (19.9% SAR, 95% CI: 14.0 – 25.7%) compared to those asymptomatic index cases (0.7%, 95% CI: 0.0 – 3.8%). It also estimated 32.5% (95% CI: 7.4 – 57.5%) of households with an index case had a secondary case. Spouses had statistically significant (P<0.001) increased secondary transmission (43.3% of spouses were infected) compared to other relatives of the index case (18.3%), and adult contacts had significant (P<0.001) increased transmission (31.0%) compared to children (15.7%).
Study strengths
The study drew upon numerous others to form the basis of its meta-analysis, which means these results are more likely to be valid estimates than if they had only found a handful. Because they had 40 studies in their pool, they were able to perform a number of further analyses to look at different dynamics of transmission, such as being a spouse versus being another relative, or differences in infection based on self-reported sex. They also controlled for major sociodemographic variables in their meta-analysis, which reduces some of the confounding between study populations and leads to less biased estimates.
Limitations
As the study conducted its various secondary analyses, the sample size dwindled, and in some cases this led to imprecise estimates with large confidence intervals. There were also differences between study samples and protocols that may result in further heterogeneity that is not wholly considered. For instance, there were differences in the study protocols for frequency and type of SARS-CoV-2 test, rates of community transmission, and differences in case definition between index case versus contacts. Asymptomatic infections were likely excluded from most of the studies, leading to a potential underestimate in the rate of transmission. Many studies could not rule out secondary infections that occurred outside the home, which, if widespread, may inflate the estimates.
Value added
This is a large meta-analysis estimating the rate of secondary infection due to household transmission, which is a pressing public health concern as reopening policies are debated.
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