To characterize the family index case for detected SARS-CoV-2 and describe testing and secondary attack rates in the family, we used individual-level administrative data of all families and all PCR tests for SARS-CoV-2 in Norway in 2020. All families with at least one parent and one child below the age of 20 who lived at the same address (N = 662,582), where at least one member, i.e. the index case, tested positive for SARS-CoV-2 in 2020, were included. Secondary attack rates (SAR7) were defined as the share of non-index family members with a positive PCR test within 7 days after the date when the index case tested positive. SARs were calculated separately for parent- and child-index cases, and for parent- and child-secondary cases. We identified 7548 families with an index case, comprising 26,991 individuals (12,184 parents, 14,808 children). The index was a parent in 66% of the cases. Among index children, 42% were in the age group 17–20 and only 8% in the age group 0–6. When the index was a parent, SAR7 was 24% (95% CI 24–25), whilst SAR7 was 14% (95% CI 13–15) when the index was a child. However, SAR7 was 24% (95% CI 20–28) when the index was a child aged 0–6 years and declined with increasing age of the index child. SAR7 from index parent to other parent was 35% (95% CI 33–36), and from index child to other children 12% (95% CI 11–13). SAR7 from index child aged 0–6 to parents was 27% (95% CI 22–33). The percent of non-index family members tested within 7 days after the index case, increased from about 20% in April to 80% in December, however, SAR7 stabilized at about 20% from May. We conclude that parents and older children are most often index cases for SARS-CoV-2 in families in Norway, while parents and young children more often transmit the virus within the family. This study suggests that whilst the absolute infection numbers are low for young children because of their low introduction rate, when infected, young children and parents transmit the virus to the same extent within the family.

为了描述检测到的 SARS-CoV-2 的家庭指数病例并描述家庭中的检测和二次发作率，我们使用了 2020 年挪威所有家庭的个人管理数据和所有 SARS-CoV-2 的 PCR 检测。至少有一名父母和一名 20 岁以下子女居住在同一地址的家庭（N = 662,582），其中至少一名成员（即指示病例）在 2020 年被检测出 SARS-CoV-2 呈阳性. 二次发作率 (SAR7) 定义为在指示病例检测呈阳性之日后 7 天内 PCR 检测呈阳性的非索引家庭成员的比例。SAR 分别针对父母和子女索引病例以及父母和儿童继发病例进行计算。我们确定了 7548 个具有指示病例的家庭，包括 26,991 人（12,184 名父母，14,808 名儿童）。在 66% 的案例中，该指数是父母。在索引儿童中，42% 的年龄在 17-20 岁之间，而只有 8% 的年龄在 0-6 岁之间。当指数为父母时，SAR7 为 24%（95% CI 24-25），而当指数为儿童时，SAR7 为 14%（95% CI 13-15）。然而，当指数为 0-6 岁儿童时，SAR7 为 24% (95% CI 20-28)，并且随着指数儿童年龄的增加而下降。从索引父母到其他父母的 SAR7 为 35% (95% CI 33-36)，从索引孩子到其他孩子的 SAR7 为 12% (95% CI 11-13)。从 0-6 岁的索引儿童到父母的 SAR7 为 27% (95% CI 22-33)。在指示病例发生后 7 天内接受检测的非索引家庭成员的百分比从 4 月的约 20% 增加到 12 月的 80%，但 SAR7 与 5 月相比稳定在 20% 左右。我们得出的结论是，父母和年龄较大的孩子最常成为挪威家庭中 SARS-CoV-2 的指示病例，而父母和年幼的孩子更经常在家庭中传播病毒。这项研究表明，虽然幼儿的绝对感染人数较低，因为他们的引入率低，但当被感染时，幼儿和父母在家庭中传播病毒的程度相同。