Related Experiment Video
Updated: Jun 15, 2026
Swabbing the Urban Environment - A Pipeline for Sampling and Detection of SARS-CoV-2 From Environmental Reservoirs
Published on: April 9, 2021
Comparison of new and emerging SARS-CoV-2 variant transmissibility through active contact testing. A comparative
Katherine M Gaskell1, Natalie El Kheir1, Mariyam Mirfendesky2
1Clinical Research Department, Faculty of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, United Kingdom.
Abstract:
Historically SARS-CoV-2 secondary attack rates (SAR) have been based on PCR positivity on screening symptomatic contacts; this misses transmission events and identifies only symptomatic contacts who are PCR positive at the time of sampling. We used serology to detect the relative transmissibility of Alpha Variant of Concern (VOC) to non-VOC SARS-CoV-2 to calculate household secondary attack rates. We identified index patients diagnosed with Alpha and non-VOC SARS-CoV-2 across two London Hospitals between November 2020 and January 2021 during a prolonged and well adhered national lockdown. We completed a household seroprevalence survey and found that 61.8% of non-VOC exposed household contacts were seropositive compared to 82.1% of Alpha exposed household contacts. The odds of infection doubled with exposure to an index diagnosed with Alpha. There was evidence of transmission events in almost all households. Our data strongly support that estimates of SAR should include serological data to improve accuracy and understanding.
Insights
The Alpha variant of SARS-CoV-2 showed higher household transmission than earlier strains. Serology revealed the Alpha variant doubled infection risk, highlighting the need for improved secondary attack rate estimates.
Area of Science:
- Virology
- Epidemiology
- Public Health
Background:
- Historically, SARS-CoV-2 secondary attack rates (SAR) relied on PCR testing of symptomatic contacts, potentially underestimating transmission.
- This method misses asymptomatic infections and transmission events occurring before symptom onset or testing.
Purpose of the Study:
- To compare the household secondary attack rates of the Alpha variant of concern (VOC) against non-VOC SARS-CoV-2 strains.
- To assess the relative transmissibility of the Alpha variant using serological data.
Main Methods:
- A household seroprevalence survey was conducted between November 2020 and January 2021 in London.
- Index patients with Alpha VOC and non-VOC SARS-CoV-2 infections were identified from two hospitals.
- Serological testing was used to determine infection status in household contacts.
Main Results:
- Household contacts exposed to the Alpha variant had a significantly higher seropositivity rate (82.1%) compared to those exposed to non-VOC strains (61.8%).
- Exposure to an index patient with the Alpha variant doubled the odds of infection among household contacts.
- Transmission events were detected in nearly all participating households.
Conclusions:
- Serological data provide a more accurate assessment of SARS-CoV-2 secondary attack rates compared to PCR-based methods alone.
- The Alpha variant demonstrated increased transmissibility within households.
- Estimates of SAR should incorporate serological data for a comprehensive understanding of viral spread.
