According to results of a prospective case-control study conducted in the SARS-CoV-2 post-vaccination era, relaxing social distancing and other protective measures may be inadvisable, especially for older frail adults or those in communities with increased deprivation. These findings were published in Lancet Infectious Diseases.
Researchers analyzed data obtained from participants who used the United Kingdom’s COVID Symptom Study App, including self-reported demographic information, COVID-19 symptoms, SARS-CoV-2 infection test results, and vaccination status. Participants were stratified into 1 of 2 cohorts: those in the first cohort had a positive COVID-19 test 14 days or more after the first but before the second vaccine dose (cases 1) and those in the second cohort had a positive test 7 days or more after the second dose (cases 2). Participants were matched in a 1:1 fashion with those who had tested negative after the first (controls 1) or second (controls 2) vaccine dose. Subsets of participants from each cohort were selected to assess the disease profile of a post-vaccination SARS-CoV-2 infection and to form cases 3 and cases 4 and were then matched with unvaccinated participants who tested positive for COVID-19 (controls 3 and controls 4).
Among the 1,240,009 app users who received the first vaccine dose and 971,504 who received the second dose, 0.5% (n=6030; cases 1) reported a positive COVID-19 test before the second dose and 0.2% (n=2370; cases 2) reported a positive test after the second dose, respectively.
Overall, more women participated in this study. In addition, cases were significantly younger than controls (P <.0001), and cases 1 had higher BMI than controls 1 (P =.0074).
An inverse relationship between increasing age and positive SARS-CoV-2 infection was observed both after the first (adjusted odds ratio [aOR], 0.94; 95% CI, 0.93-0.95; P <.001) and second (aOR, 0.93; 95% CI, 0.92-0.95; P <.0001) vaccine dose.
A positive relationship was observed among participants in cases 1 in regard to frailty among older adults (aOR, 1.93; 95% CI, 1.50-2.48; P <.0001), with additional risk factors in regard to kidney disease (OR, 1.95; 95% CI, 1.14-3.31; P =.014), heart disease (OR, 1.30; 95% CI, 1.03-1.65; P =.031), and lung disease (OR, 1.27; 95% CI, 1.02-1.59; P =.030).
After stratification by socioeconomic status, the researchers noted participants living in areas with increased deprivation were associated with an increased risk for post-vaccination infection after receiving the first vaccine dose (OR, 1.11; 95% CI, 1.01-1.23; P =.039).
The average duration of infection among participants in cases 1 and cases 2 were 73 ± 44 and 51 ± 30 days, respectively.
An increased duration of COVID-19 symptoms (³28 days) was less likely to occur after receiving the second vaccine dose (aOR, 0.51; 95% CI, 0.32-0.82; P =.0060), and participants who received 2 vaccine does were less likely to experience 5 or more co-occurring symptoms.
This study was limited by its skewed population of individuals who chose to participate, its inclusion of more women than men, and its lack of participants living in areas of deprivation.
The researchers noted that post-vaccination infection was more likely to occur among frail, older adults and those who lived in more deprived areas; indicating that relaxing personal protective measures may result in new infections among vulnerable populations even after vaccination.
Disclosure: Some author(s) declared affiliations with industry. Please refer to the original article for a full list of disclosures.
Antonelli M, Penfold RS, Merino J, et al. Risk factors and disease profile of post-vaccination SARS-CoV-2 infection in UK users of the COVID Symptom Study app: a prospective, community-based, nested, case-control study. Lancet Infect Dis. 2021;S1473-3099(21)00460-6. doi:10.1016/S1473-3099(21)00460-6