Impact of Delta Variant and Vaccination on SARS-CoV-2 Secondary Attack Rate Among Household Close Contacts

Oon Tek Ng; Vanessa Koh; Calvin J Chiew; Kalisvar Marimuthu; Natascha May Thevasagayam; Tze Minn Mak; Joon Kiat Chua; Shannen Si Hui Ong; Yong Kai Lim; Zannatul Ferdous; Alifa Khairunnisa Bte Johari; Mark I-Cheng Chen; Sebastian Maurer-Stroh; Lin Cui; Raymond Tzer Pin Lin; Kelvin Bryan Tan; Alex R Cook; Prof Yee-Sin Leo; Prof Vernon Jm Lee

doi:10.1016/j.lanwpc.2021.100299

Impact of Delta Variant and Vaccination on SARS-CoV-2 Secondary Attack Rate Among Household Close Contacts

Lancet Reg Health West Pac. 2021 Dec:17:100299. doi: 10.1016/j.lanwpc.2021.100299. Epub 2021 Nov 1.

Authors

Oon Tek Ng^{1

2

3}, Vanessa Koh^{1

2}, Calvin J Chiew⁴, Kalisvar Marimuthu^{1

2

5}, Natascha May Thevasagayam^{1

2}, Tze Minn Mak¹, Joon Kiat Chua⁶, Shannen Si Hui Ong⁶, Yong Kai Lim⁶, Zannatul Ferdous^{1

2}, Alifa Khairunnisa Bte Johari^{1

2}, Mark I-Cheng Chen^{1

4

7}, Sebastian Maurer-Stroh^{1

8

9

10}, Lin Cui¹, Raymond Tzer Pin Lin^{1

5}, Kelvin Bryan Tan^{6

7

11}, Alex R Cook⁷, Prof Yee-Sin Leo^{1

2

3

5

7}, Prof Vernon Jm Lee^{4

7}

Affiliations

¹ National Centre for Infectious Diseases, Singapore.
² Tan Tock Seng Hospital, Singapore.
³ Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore.
⁴ Communicable Diseases Division, Ministry of Health, Singapore.
⁵ Yong Loo Lin School of Medicine, National University of Singapore and National University Health System, Singapore.
⁶ Ministry of Health, Singapore.
⁷ Saw Swee Hock School of Public Health, National University of Singapore and National University Health System, Singapore.
⁸ Bioinformatics Institute, Agency for Science Technology and Research, Singapore.
⁹ Infectious Diseases Labs, Agency for Science Technology and Research, Singapore.
¹⁰ Department of Biological Sciences, National University of Singapore, Singapore.
¹¹ Centre for Regulatory Excellence, Duke-NUS Medical School.

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.