Increased Secondary Attack Rates among the Household Contacts of Patients with the Omicron Variant of the Coronavirus Disease 2019 in Japan

Tsuyoshi Ogata, Hideo Tanaka, Emiko Tanaka, Natsumi Osaki, Etsuko Noguchi, Yukino Osaki, Ayane Tono and Koji Wada
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Tsuyoshi Ogata: Itako Public Health Center of Ibaraki Prefectural Government, Itako 311-2422, Japan
Hideo Tanaka: Public Health Center of Neyagawa City Government, Neyagawa 572-0838, Japan
Emiko Tanaka: Itako Public Health Center of Ibaraki Prefectural Government, Itako 311-2422, Japan
Natsumi Osaki: Itako Public Health Center of Ibaraki Prefectural Government, Itako 311-2422, Japan
Etsuko Noguchi: Itako Public Health Center of Ibaraki Prefectural Government, Itako 311-2422, Japan
Yukino Osaki: Itako Public Health Center of Ibaraki Prefectural Government, Itako 311-2422, Japan
Ayane Tono: Itako Public Health Center of Ibaraki Prefectural Government, Itako 311-2422, Japan
Koji Wada: Department of Public Health, Faculty of Medicine, International University of Health and Welfare, Tokyo 107-8402, Japan

IJERPH, 2022, vol. 19, issue 13, 1-11

Abstract: This study investigated the household secondary attack rate (HSAR) of patients with coronavirus disease (COVID-19) during the omicron variant-dominant period. The HSAR of COVID-19 cases during the omicron variant-dominant period (4–20 January 2022) was calculated and compared with the delta variant-dominant period (20 August to 7 November 2021) in Itako, Japan. In Itako, all 47 and 119 samples tested during the omicron and delta variant-dominant periods were negative and positive, respectively, for the L452R mutation. We used a generalized estimating equation regression model. The HSAR was 31.8% (95% confidence interval (CI) 27.7–36.2) for 456 household contacts during the omicron variant-dominant period; it was higher than that during the delta variant-dominant period (25.2%) (adjusted risk ratio [aRR] 1.61, CI 1.13–2.28). During the omicron variant-dominant period, HSAR was lower for the household contacts of completely vaccinated index patients (27.3%) than for contacts of other index patients (41.2%) (vaccine effectiveness for infectee 0.43, 95% CI 0.16–0.62) and was significantly higher for female contacts than for male contacts (36.2% vs. 26.1%; aRR 1.29, 95% CI 1.01–1.65). The HSAR was significantly higher during the omicron variant-dominant period than the delta variant-dominant period. The vaccination of index patients might protect household contacts.

Keywords: COVID-19; Omicron variant; SARS-CoV-2; household transmission; secondary attack rate; vaccine effectiveness; sex; Delta variant; Japan (search for similar items in EconPapers)
JEL-codes: I I1 I3 Q Q5 (search for similar items in EconPapers)
Date: 2022
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