Table 3. Secondary transmission risk.

The secondary attack rate (SAR) from an infected parent to susceptible children in the household by parent vaccination status. The unit of observation for this analysis consisted of households in which a parent (the index parent) was infected with SARS-CoV-2. The exposure was the vaccination status of the index parent. The outcome was infection of at least one child in the household at days 3 to 8 after diagnosis of the index parent. To maintain a well-defined point of entry of the infection, we excluded households in which the parent who is not the index parent or a child was diagnosed on days 0 to 2 after diagnosis of the index parent. During the early period, full vaccination was defined as the receipt of two doses at least 7 days prior (compared with no vaccination), and the dominant variant was Alpha. During the late period, full vaccination was defined as receipt of a third dose at least 7 days prior (compared with receipt of only two doses at least 5 months prior), and the dominant variant was Delta. The adjusted estimate was derived from a logistic regression model adjusted for all the household-level characteristics and the vaccination status of the nonindex parent.

Characteristic	Early period	Late period
SAR from a vaccinated or boosted parent (%)	9.0%	9.3%
SAR from an unvaccinated or unboosted parent (%)	24.7%	31.1%
1 − adjusted odds ratio (95% CI)	72.1% (36.6%, 89.3%)	>79.6% (55.9%, 91.8%)