In studies in line with the WHO Unity Studies Household Transmission Investigations (HHTI) protocol, researchers aimed to calculate the household secondary infection attack rate (hSAR) of SARS-CoV-2. By PRISMA 2020 recommendations, they performed a systematic review and meta-analysis. Investigators looked for “Unity-aligned” First Few X cases (FFX) and HHTIs published between 1 December 2019 and 26 July 2021 in Medline, Embase, Web of Science, Scopus, and medRxiv/bioRxiv WHO Unity Studies collaborators shared preliminary results that had been standardized (to 1 October 2021). Study group used a custom tool to evaluate the inquiry’s methodological quality. From the raw data, values for hSAR and 95% CIs were taken or derived. By visually examining the overlap of CIs on forest plots, heterogeneity was evaluated and measured. About 80 papers (64 from databases and 16 from Unity Studies collaborators) were selected for the systematic review out of the 9,988 records, and 62 were included in the primary meta-analysis. I2 values remained less than 99% in subgroup analyses, indicating strong, unexplained heterogeneity and prompting a decision not to provide pooled hSAR estimates. hSAR point estimates varied from 2% to 90% (95% prediction interval: 3%–71%; I2=99.7%). For the ongoing and early characterization of new infectious diseases, FFX and HHTI remain essential epidemiological techniques. To direct evidence-based pandemic preparedness and response actions for SARS-CoV-2, influenza, and future new respiratory viruses, it was imperative to encourage standardization in planning, conducting, and analysis as well as clear and thorough reporting of FFX and HHTIs in time and place.