COVID-19 Transmission Within Danish Households: A Nationwide Study from Lockdown to Reopening

Abstract

Background The Covid-19 pandemic is one of the most serious global public health threats in recent times. Understanding transmission of SARS-CoV-2 is of utmost importance to be able to respond to outbreaks and take action against spread of the disease. Transmission within the household is a concern, especially because infection control is difficult to apply within the household domain. Methods We used comprehensive administrative register data from Denmark, comprising the full population and all COVID-19 tests, to estimate household transmission risk and attack rate. Results We studied the testing dynamics for COVID-19 and found that the day after receiving a positive test result within the household, 35% of potential secondary cases were tested and 13% of these were positive. After a primary case in 6,782 households, 82% of potential secondary cases were tested within 14 days, of which 17% tested positive as secondary cases, implying an attack rate of 17%. Among primary cases, those aged 0-24 were underrepresented when compared with the total population. We found an approximately linearly increasing relationship between attack rate and age. We investigated the transmission risk from primary cases by age, and found an increasing risk with age of primary cases for adults, while the risk seems to decrease with age for children. Conclusions Although there is an increasing attack rate and transmission risk of SARS-CoV-2 with age, children are also able to transmit SARS-CoV-2 within the household.

Household Transmission of SARS-CoV-2

“…Secondary attack rates were not significantly different when restricting to 38 studies 19 , 20 , 22 , 23 , 26 , 27 , 28 , 29 , 30 , 31 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 42 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 54 , 55 , 56 , 57 , 60 , 62 , 63 , 65 , 67 , 68 , 69 , 72 with low or moderate risk of bias (15.6%; 95%, 12.8%-18.5%) (eFigure 4 in the Supplement ). There were no significant differences in secondary attack rates between 21 studies in China 22 , 27 , 31 , 36 , 37 , 39 , 45 , 46 , 48 , 58 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 70 , …”

“…We identified 54 relevant published studies that reported household secondary transmission, with 77 758 participants (eTable 1 in the Supplement ). 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 A total of 16 of 54 studies (29.6%) were at high risk of bias, 27 (50.0%) were moderate, and 11 (20.4%) were low (eTable 2 in the Supplement ). Lower quality was attributed to studies with 1 or fewer test per contact (35 studies [64.8%]), small sample sizes (31 [57.4%]), a...…”

“…Figure 1 summarizes secondary attack rates for 44 studies 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 28 , 29 , 30 , 32 , 33 , 34 , 35 , 36 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 59 , 61 , 62 , 63 , 65 , 66 , 67 , 69 , 70 of household contacts and 10 of family contacts. 26 , 31 , 37 , 45 , 58 , 60 , 65 , 68 , 71 , 72 Estimated mean secondary attack rate for household contacts was 16.4% (95% CI, 13.4%-19.6%) and family contacts was 17.4% (95% CI, 12.7%-22.5%).…”

“…A number of studies examined factors associated with susceptibility of household contacts to infection (eTable 6 in the Supplement ). Age was the most examined covariate, with most studies 20 , 29 , 36 , 37 , 38 , 39 , 45 , 46 , 48 , 49 , 55 , 63 , 65 , 68 reporting lower secondary transmission of SARS-CoV-2 to child contacts than adult contacts. In 5 studies, 20 , 36 , 39 , 48 , 49 individuals older than 60 years were most susceptible to SARS-CoV-2 infection.…”

“…Contact age was not associated with susceptibility in 9 studies, 26,28,32,44,47,58,66,67,70 although these were typically less powered to detect a difference. Figure 3 summarizes 15 studies 22,26,29,37,39,42,44,45,47,49,55,59,60,63,65 The second most examined factor was sex of exposed contacts, which was not associated with susceptibility for most studies 20,22,26,32,36,39,44,45,[47][48][49]58,[65][66][67]70 except 3. Spouse relationship to index case was associated with secondary infection in 4 studies 26,45,46,58 of 6 in which this was examined.…”