Please use this identifier to cite or link to this item: https://doi.org/10.1016/j.lanwpc.2021.100299
Title: Impact of Delta Variant and Vaccination on SARS-CoV-2 Secondary Attack Rate Among Household Close Contacts
Authors: Ng, Oon Tek 
Koh, Vanessa
Chiew, Calvin J
Marimuthu, Kalisvar 
Thevasagayam, Natascha May 
Mak, Tze Minn
Chua, Joon Kiat
Ong, Shannen Si Hui
Lim, Yong Kai
Ferdous, Zannatul
Johari, Alifa Khairunnisa Bte
Chen, Mark I-Cheng 
Maurer-Stroh, Sebastian 
Cui, Lin 
Lin, Raymond Tzer Pin 
Tan, Kelvin Bryan
Cook, Alex R 
Leo, Yee-Sin 
Lee, Vernon JM 
Keywords: Science & Technology
Life Sciences & Biomedicine
Health Care Sciences & Services
Public, Environmental & Occupational Health
TRANSMISSION
CHILDREN
Issue Date: 1-Nov-2021
Publisher: ELSEVIER
Citation: Ng, Oon Tek, Koh, Vanessa, Chiew, Calvin J, Marimuthu, Kalisvar, Thevasagayam, Natascha May, Mak, Tze Minn, Chua, Joon Kiat, Ong, Shannen Si Hui, Lim, Yong Kai, Ferdous, Zannatul, Johari, Alifa Khairunnisa Bte, Chen, Mark I-Cheng, Maurer-Stroh, Sebastian, Cui, Lin, Lin, Raymond Tzer Pin, Tan, Kelvin Bryan, Cook, Alex R, Leo, Yee-Sin, Lee, Vernon JM (2021-11-01). Impact of Delta Variant and Vaccination on SARS-CoV-2 Secondary Attack Rate Among Household Close Contacts. LANCET REGIONAL HEALTH-WESTERN PACIFIC 17. ScholarBank@NUS Repository. https://doi.org/10.1016/j.lanwpc.2021.100299
Abstract: Background: Impact of the Delta variant and vaccination on SARS-CoV-2 transmission remains unclear. In Singapore, quarantine of all close contacts, including entry and exit PCR testing, provided the opportunity to determine risk of infection by the Delta variant compared to other variants, vaccine efficacy against SARS-CoV-2 acquisition, symptomatic or severe COVID-19, and risk factors associated with SARS-CoV-2 acquisition and symptomatic disease. Methods: This retrospective cohort study included all close contacts between September 1, 2020 and May 31, 2021. Regardless of symptoms, all were quarantined for 14 days with entry and exit PCR testing. Household contacts were defined as individuals who shared a residence with a Covid-19 index case. Secondary attack rates among household close contacts of Delta variant-infected indexes and other variant-infected indexes were derived from prevalence of diagnosed cases among contacts. Relative risk ratios and bootstrapping at the cluster level was used to determine risk of infection by the Delta variant compared to other variants and vaccine efficacy against SARS-CoV-2 acquisition, symptomatic or severe COVID-19. Logistic regression using generalized estimating equations was used to determine risk factors associated with SARS-CoV-2 acquisition and symptomatic disease Findings: Of 1024 household contacts linked to 301 PCR-confirmed index cases, 753 (73.5%) were linked to Delta-infected indexes and 248 (24.2%) were exposed to indexes with other variants. Household secondary attack rate among unvaccinated Delta-exposed contacts was 25.8% (95% boostrap confidence interval [BCI] 20.6–31.5%) compared with 12.9% (95%BCI 7.0–20.0%) among other variant-exposed contacts. Unvaccinated Delta-exposed contacts were more likely to be infected than those exposed to other variants (Relative risk 2.01, 95%CI 1.24–3.84). Among Delta-exposed contacts, complete vaccination had a vaccine effectiveness of 56.4% (95%BCI 32.6–75.8%) against acquisition, 64.1% (95%BCI 37.8–85.4%) against symptomatic disease and 100% against severe disease. Among Delta-exposed contacts, vaccination status (adjusted odds ratio [aOR] 0.33, 95% robust confidence interval [RCI] 0.17–0.63) and older age of the index (aOR 1.20 per decade, 95%RCI 1.03–1.39) was associated with increased risk of SARS-CoV-2 acquisition by the contact. Vaccination status of the index was not associated with a statistically-significant difference for contact SARS-CoV-2 acquisition (aOR 0.73, 95%RCI 0.38–1.40). Interpretation: Increased risk of SARS-CoV-2 Delta acquisition compared with other variants was reduced with vaccination. Close-contacts of vaccinated Delta-infected indexes did not have statistically significant reduced risk of acquisition compared with unvaccinated Delta-infected indexes.
Source Title: LANCET REGIONAL HEALTH-WESTERN PACIFIC
URI: https://scholarbank.nus.edu.sg/handle/10635/228520
ISSN: 26666065
DOI: 10.1016/j.lanwpc.2021.100299
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