Abstract
Background: In May, 2021, the delta (B.1.617.2) SARS-CoV-2 variant became dominant in the UK, superseded by the omicron (B.1.1.529) variant in December, 2021. The delta variant is associated with increased transmissibility compared with the alpha variant, which was the dominant variant in the UK between December, 2020, and May, 2021. To understand transmission and the effectiveness of interventions, we aimed to investigate whether the delta variant generation time (the interval between infections in infector–infectee pairs) is shorter—ie, transmissions are happening more quickly—than that of the alpha variant.
Methods: In this epidemiological analysis, we analysed transmission data from an ongoing UK Health Security Agency (UKHSA) prospective household study. Households were recruited to the study after an index case had a positive PCR test and genomic sequencing was used to determine the variant responsible. By fitting a mathematical transmission model to the data, we estimated the intrinsic generation time (which assumes a constant supply of susceptible individuals throughout infection) and the household generation time (which reflects realised transmission in the study households, accounting for susceptible depletion) for the alpha and delta variants.
Findings: Between February and August, 2021, 227 households consisting of 559 participants were recruited to the UKHSA study. The alpha variant was detected or assumed to be responsible for infections in 131 households (243 infections in 334 participants) recruited in February–May, and the delta variant in 96 households (174 infections in 225 participants) in May–August. The mean intrinsic generation time was shorter for the delta variant (4·7 days, 95% credible interval [CI] 4·1–5·6) than the alpha variant (5·5 days, 4·7–6·5), with 92% posterior probability. The mean household generation time was 28% (95% CI 0–48%) shorter for the delta variant (3·2 days, 95% CI 2·5–4·2) than the alpha variant (4·5 days, 3·7–5·4), with 97·5% posterior probability.
Interpretation: The delta variant transmits more quickly in households than the alpha variant, which can be attributed to faster depletion of susceptible individuals in households and a possible decrease in the intrinsic generation time. Interventions such as contact tracing, testing, and isolation might be less effective if transmission of the virus occurs quickly.
Funding: National Institute for Health Research, UK Health Security Agency, Engineering and Physical Sciences Research Council, and UK Research and Innovation.
| Original language | English |
|---|---|
| Pages (from-to) | 603-610 |
| Number of pages | 8 |
| Journal | The Lancet Infectious Diseases |
| Volume | 22 |
| Issue number | 5 |
| Early online date | 14 Feb 2022 |
| DOIs | |
| Publication status | Published - 20 Apr 2022 |
Bibliographical note
Funding Information: National Institute for Health Research, UK Health Security Agency, Engineering and Physical Sciences Research Council, and UK Research and Innovation.Data collection for this study was done by the UK Health Security Agency (UKHSA; an executive agency of the UK Department of Health), as part of the COVID-19 response. WSH was funded by an Engineering and Physical Sciences Research Council Excellence Award for his doctoral studies (EP/R513295/1). EM receives support from the National Institute for Health Research (NIHR) Health Protection Research Unit in Immunisation at the London School of Hygiene & Tropical Medicine in partnership with the UKHSA (NIHR200929). SF receives funding from the Wellcome Trust (210758/Z/18/Z). RNT receives support from UK Research and Innovation (EP/V053507/1). We thank Matt Keeling for helpful discussions about this research. We also thank household members who took part in this study, the UKHSA nursing staff who recruited and followed up the households, the laboratory staff who tested the swabs, and administrative staff who arranged for the delivery and collection of testing kits from households.
EM, PW, and NJA are employees of the UKHSA. The UKHSA was involved in the design of the household study, and collecting, accessing, and verifying the data.
Other funders had no role in the study design, data collection, data analysis, data interpretation, writing of the report, or the decision to submit the paper for
publication.
We declare no competing interests. WSH, SF, and RNT report participation in discussions of the UK Government’s Scientific Pandemic Influenza Group on Modelling, Operational subgroup
Open Access: This is an Open Access article under the CC BY 4.0 license.
Publisher Copyright: Crown Copyright © 2022 Published by Elsevier Ltd.
Citation: William S Hart, Elizabeth Miller, Nick J Andrews, Pauline Waight, Philip K Maini, Sebastian Funk, Robin N Thompson, Generation time of the alpha and delta SARS-CoV-2 variants: an epidemiological analysis, The Lancet Infectious Diseases,
Volume 22, Issue 5, 2022, Pages 603-610, ISSN 1473-3099.
DOI: https://doi.org/10.1016/S1473-3099(22)00001-9.
