A new study in Nature Communications suggests that both prior infection with COVID-19 and prior vaccination with mRNA vaccines can limit the secondary attack rate of SARS-CoV-2.
Immunity from prior infections is stronger at limiting contagiousness, but fades quickly, compared to immunity from vaccines, which was longer-lasting. And SARS-CoV-2 infections, of course, come with much higher risks of poor outcomes than do COVID vaccines.
The study was based on attack rates seen among 50,973 index cases and 111,674 declared contacts of infected people in Geneva from June 2020 to March 2022. The study period covered four variants of concerns (VOCs), and was based on case contacts' COVID-19 test results and not biological samples.
Most contacts were household members
The mean number of declared contacts per infected person was 2.2, with a net decrease during the Omicron period (1.6 mean contacts per index case). Overall, children and younger adults were overrepresented as index cases in the study, the authors said.
"Index cases were at 73% adults between 18 and 64 years, 22% children, and 4.6% adults older than 65 years. The proportion of children for the index cases tripled between the EU1 wave (11%) and the Delta wave (38%)," the authors wrote.
Ninety-four percent of index cases reported symptoms, including 58% with a cough.
Household contacts made up 63% of contacts identified in the study, and this percentage increased up to 77% during the Omicron wave.
Only 2% of index cases were vaccinated during the Alpha wave, whereas 52% were vaccinated during the Omicron wave, of which 25% had their last dose more than 6 months before the infection.
19% of contacts tested positive
Among 111,674 contacts, 46,417 took a COVID-19 test during the 10 days following the date of the last contact with the index case, and 21,435 had a positive test result, a raw 19.2% secondary attack rate (SAR).
The raw SAR was from 16% to 27% for each of the four VOCs, from 16.4% during the initial wild-type wave, 20.9% during the Alpha wave, 16.7% during the Delta wave, and 26.3% during the Omicron wave.
Previous COVID-19 infection in the index case reduced the SAR by 10.5 adjusted percent points (pp) (95% confidence interval [CI], 7.0 to 14.0) during the wild-type wave, 8.6 pp during the Alpha wave (4.3 to 12.8), 11.3 pp during the Delta wave (8.6 to 14.0), and 4.3 pp during Omicron (1.3 to 7.3).
The effect of previous infection was stronger for the contacts, with a greater effect when the date of infection was less than 6 months before the index-contact date.
"The effect of previous infection was stronger for the contacts, with a greater effect when the date of infection was less than 6 months before the index-contact date," the authors said.
Recent vaccination tied to lower attack rate
A vaccinated index case-patient was associated with a lower SAR across VOCs, mainly when the last dose of vaccination was less than 6 months before the index-contact date (SAR change of −5.1 pp [−9.9 to −0.3] during Alpha, −5.4 pp [−6.7 to −4.1] during Delta and −6.5 pp [−8.2 to −4.8] during Omicron).
The vaccination of recent vaccination of contacts had a strong protective effect for Alpha (−12.9 pp [−15.4 to −10.5]) and Delta variants (−9.5 pp [−10.6 to −8.5]), the authors said.
If vaccination occurred more than 6 months before the last meeting between index and contact, it did not have a significant effect during the Delta variant, and even was tied to an increased SAR during the Omicron wave of infection (increase of 13.3 pp [11.2 to 15.3]).
"The reduction of infectiousness conferred by vaccination appears to wane less in time and to be less sensitive to variant changes than the decrease of infection susceptibility, making this effect the major contribution of vaccination to the reduction of SARS-CoV-2 propagation for Omicron," the authors concluded.
The study authors also note that while natural infection seemed to have a bigger impact on SAR initially, it came with inherent risks.
"The risks associated with infection are significant, particularly for frail people. We also know that there are cumulative risks associated with multiple infections, particularly cardiac, or neurological risks," said senior autho Delphine Courvoisier, PhD, in a University of Geneva press release.