COVID-19 vaccines appear to help prevent transmission between household contacts, with secondary attack rates dropping from 31% to 11% if the index patient was fully vaccinated, according to a Eurosurveillance study yesterday. The population-based data looked at the Netherlands from February to May, when the Alpha variant (B117) was dominant and the available vaccines were by Pfizer/BioNTech, AstraZeneca/Oxford, Moderna, and Johnson & Johnson.
"This finding underscores the importance of full vaccination of close contacts of vulnerable persons," the researchers write. "Further research is needed to determine whether the observed differences between the different vaccines are due to the small sample size or have real public health relevance."
Vaccines have 71% effectiveness against transmission
The researchers looked at 113,582 adult index cases and 253,168 close contacts of all ages; 5,394 fell under both categories. (A close contact was also classified as an index case when they tested positive.) Less than 1% (0.5%) of index cases were fully vaccinated, while 1.8% were partially vaccinated. For close contacts, 2.1% of were fully vaccinated and 1.7% were partially vaccinated.
The secondary attack rate was 31% for household members exposed to unvaccinated index patients but 11% if the index patient was fully vaccinated, making the adjusted vaccine effectiveness against transmission (aVET) 71% (95% confidence interval [CI], 63% to 77%). By vaccine type, VET was estimated to be 58% for the AstraZeneca/Oxford vaccine, 70% for Pfizer/BioNTech, 88% for Moderna, and 77% for Johnson & Johnson. Partial vaccination only resulted in an aVET between 15% and 51%.
For close contacts who were not household members, the researchers say VET seemed to be 22% (95% CI, -5% to 43%), which they suggest could be due to misclassification of the index case.
"Although we tried to minimise misclassification of indexes and contacts by excluding index cases infected at home and contacts with symptom onset before or at the same time as the index, it is plausible that in some instances, the transmission route was reversed or transmission occurred [through] another source (especially for non-household contacts)," the researchers write, saying this could cause an underestimation of VET.
On the other hand, they continue, "Because our analysis on household contacts was restricted to notified index cases not infected at home, probably most of these index cases sought testing because they had symptoms. Symptomatic cases may have been misclassified as index cases in a household, where in reality an asymptomatic household member was the source of transmission to the supposed index case and a third household member. If vaccinees are more likely to be asymptomatic, this source of misclassification may result in an overestimation of the VET."
Overall, the researchers say adjusted vaccine effectiveness (aVE) for fully vaccinated household contacts after confirmed exposure was 75% and 79% for other close contacts (95% CIs, 72% to 78% and 74% to 83%, respectively).