COVID vaccine markedly cuts household transmission, studies show

Older couple with vaccine bandages on their arms
Older couple with vaccine bandages on their arms

lucigerma / iStock

A pair of studies in yesterday in Science show the substantial benefit of COVID-19 vaccination to household contacts in Israel, including unvaccinated children. But one of the studies highlighted waning protection over time.

Two vaccinated parents better than 1 at curbing spread

A team led by Clalit Health Services researchers in Tel Aviv studied households consisting of two COVID-19–naïve parents and their unvaccinated children younger than 16 years from Jan 17 to Mar 28, 2021, and households with children younger than 11 years from Jul 11 to Sep 30, 2021.

The early period was characterized by dominance of the Alpha (B117) variant and was before the availability of third COVID-19 vaccine doses. During the latter period, the Delta (B1617.2) variant was dominant, and booster doses were extended to the entire population in August.

In the early period, 400,733 unvaccinated children from 155,305 households participated. Median age was 6 years. The later period involved 181,307 unvaccinated children from 76,621 households. Median age in that period was 5 years.

The researchers also estimated the decline in risk of a vaccinated parent contracting COVID-19 (direct vaccine effectiveness), and the drop in risk that an infected vaccinated parent would then infect an unvaccinated child (household infectiousness).

Parental vaccination (one parent) with two doses of the Pfizer/BioNTech COVID-19 vaccine was tied to a 26.0% (95% confidence interval [CI], 14.0% to 36.2%) lower risk to their children than no vaccination in the early period. In the later period, having one parent who completed the primary vaccine series and received a booster dose was associated with a 20.8% (95% CI, 11.4% to 29.1%) lower risk.

Among this group, the vaccine effect was similar among age-groups and household sizes. For instance, adjusted vaccine effectiveness was 67.1% (95% CI, 52.4% to 77.3%) for a household of three in which both parents were vaccinated and 62.9% (95% CI, 44.2% to 75.4%) for a household of seven with two vaccinated parents.

Having both parents vaccinated with two doses and a booster was linked to a 71.7% (95% CI, 68.6% to 74.6%) and 58.1% (95% CI, 53.1% to 62.6%) reduced risk in the early and late periods, respectively, versus only two doses.

Among this group, the vaccine effect differed by age-group and household size. For example, adjusted vaccine effectiveness was 65.9% (95% CI, 56.7% to 73.2%) for a child 0 to 2 years old with two boosted parents, while it was 55.5% (95% CI, 48.6% to 61.6%) for a child of 7 to 11 years with two boosted parents.

Analysis of the direct effect of two doses of vaccine on the risk of parental infection estimated a 94.4% (95% CI, 93.2% to 95.4%) reduction in the risk of COVID-19 infection during the early period and a 86.3% (95% CI, 83.4% to 88.6%) lower risk of documented infection during the late period.

Two doses of vaccine in an infected parent who was vaccinated was tied to 72.1% (95% CI, 36.6% to 89.3%) lower odds of infection of one or more unvaccinated children from that parent in the early period and 79.6% (95% CI, 55.9% to 91.8%) lower odds of spread from an infected boosted parent to one or more unvaccinated children in the late period.

The study authors said that the findings underscore the importance of increasing vaccine uptake among the vaccine-eligible population to protect those who cannot be vaccinated. "Parental vaccination confers substantial protection to children residing in the same household," they wrote.

Estimated 89.4% lower risk of infection

Another study, this one by Yale University and Maccabi Healthcare Services in Tel Aviv, used a model to estimate the effectiveness of the Pfizer COVID-19 vaccine against transmission to adult or child household contacts before and after the emergence of the Delta variant in Israel, from Jun 1, 2020, to Jul 28, 2021.

Of 2,472,502 Maccabi Health Services members from 1,327,647 households, 1,471,386 received two COVID-19 vaccine doses by Jul 28, 2021, before third doses became widely available in Israel. Most households (60%) had a single household member, who was infected in 62,295 (7.8%) of 797,170 households.

There were 202,298 COVID-19 infections (8.2% of the population) over the study period. Of them, 6,483 infections occurred in those who had received two vaccine doses, while 186,975 occurred in the unvaccinated (unadjusted risk ratio, 6.6%).

COVID-19 vaccination reduced the risk of infection by 89.4% (95% CI, 88.7% to 90.0%), falling to 58.3% more than 90 days after the second dose. Vaccine effectiveness against infectiousness was 23.0% (95% CI, -11.3% to 46.7%) 10 to 90 days after the second dose but fell to 6.9% after 90 days.

Total vaccine effectiveness (reduction in risk of infection and infectiousness) was 91.8% (95% CI, 88.1% to 94.3%) but declined to 61.1% (95% CI, 5.2% to 84.1%) 90 days after the second dose. Transmission from one household member to another was lower when the index patient was vaccinated against COVID-19 before the Delta period.

After the emergence of Delta, estimated vaccine effectiveness was 72.0% (95% CI, 65.9% to 77.0%) within 10 to 90 days of the second dose and 40.2% (95% CI, 37.6% to 42.6%) after 90 days. Similarly, total vaccine effectiveness was 65.6% (95% CI, 4.9% to 87.6%) within 10 to 90 days and 24.2% (95% CI, 9.0% to 36.9%) after 90 days.

The reduction of infection among children younger than 12 years old exposed to an infected vaccinated versus unvaccinated household member was 41.0% (95% CI, −13.7% to 69.4%) 10 to 90 days after the second dose.

Children younger than 12 years were at lower risk of infection from both the community and household members, while adults 40 to 64 years and those 65 and older had a lower risk of infection from the community and a higher risk of infection within the household compared with those 12 to 39 years old. A sensitivity analysis suggested that children were slightly less infectious than adults.

"Our analysis suggests that while vaccines provide good protection against coronavirus infection, this protection wanes over time," senior author Virginia Pitzer, ScD, of Yale, said in a Yale press release.

"Moreover, vaccinated people who got infected with the delta variant were just as infectious as unvaccinated cases. This emphasizes the need for booster doses and for people who are infected to isolate regardless of whether or not they are vaccinated."

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