Background Understanding the clinical spectrum of SARS-CoV-2 infection, including the asymptomatic fraction, is important as asymptomatic individuals are still able to infect other individuals and contribute to ongoing transmission. The WHO Unity Household transmission investigation (HHTI) protocol provides a platform for the prospective and systematic collection of high-quality clinical, epidemiological, serological, and virological data from SARS-CoV-2 confirmed cases and their household contacts. These data can be used to understand key severity and transmissibility parameters — including the asymptomatic proportion — in relation to local epidemic context and help inform public health response.

Methods We aimed to estimate the asymptomatic proportion of SARS-CoV-2 Omicron-variant infections in Unity-aligned HHTIs. We conducted a systematic review and meta-analysis in alignment with the PRISMA 2020 guidelines and registered our systematic review on PROSPERO (CRD42022378648). We searched EMBASE, Web of Science, MEDLINE, and bioRxiv and medRxiv from 1 November 2021 to 22 August 2023.

Results We identified 8,368 records, of which 98 underwent full text review. We identified only three studies for data extraction, with substantial variation in study design and corresponding estimates of the asymptomatic proportion. As a result, we did not generate a pooled estimate or I2 metric.

Conclusions The limited number of quality studies that we identified highlights the need for improved preparedness and response capabilities to facilitate robust HHTI implementation, analysis and reporting, to better inform national, regional and global risk assessments and policy making.

Key messages

Estimates for the asymptomatic proportion of SARS-CoV-2 Omicron-variant infections are highly heterogeneous

We assessed the proportion of SARS-CoV-2 Omicron-variant infections among household contacts, who were followed prospectively and systematically, per the WHO Unity household transmission investigation protocol.

Given the small number of studies with sufficient data and the observed heterogeneity in the asymptomatic proportion point estimates, we did not provide a pooled estimate of the asymptomatic proportion.

Fit-for-purpose study designs, and improved reporting, are necessary for robust estimation of epidemiological characteristics from household studies and their interpretation.

Ongoing assessment of the asymptomatic proportion of SARS-CoV-2 infection is critical to inform ongoing public health response options such as testing strategies to detect infections and isolation guidance for close contacts.

The authors have declared no competing interest.

AJM and DJP are supported by funding from Australian National Health and Medical Research Council Partnership (#1195895).

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