COVID-19 vaccines show excellent efficacy in clinical trials and effectiveness in real-world data, but some people still become infected with SARS-CoV-2 after vaccination. A UK study aimed to identify risk factors for post-vaccination SARS-CoV-2 infection and describe the characteristics of post-vaccination illness. The study results were just released in The Lancet.
This prospective, community-based, nested, case-control study used self-reported data (eg, on demographics, geographical location, health risk factors and COVID-19 test results, symptoms and vaccinations) from UK-based, adult (≥18 years) users of the COVID Symptom Study mobile phone app.
For the risk factor analysis, cases had received a first or second dose of a COVID-19 vaccine between Dec 8, 2020, and July 4, 2021; had either a positive COVID-19 test at least 14 days after their first vaccination (but before their second; cases 1) or a positive test at least 7 days after their second vaccination; and had no positive test before vaccination.
Vaccination against SARS-CoV-2 is a leading strategy to change the course of the COVID-19 pandemic worldwide. The UK was the first country to authorize a vaccine against SARS-CoV-2, with three licensed as of July, 2021: BNT162b2 (tozinameran; Pfizer–BioNTech), mRNA-1273 (elasomeran; Moderna), and ChAdOx1 nCoV-19 (Oxford–AstraZeneca), each with good efficacy in phase 3 clinical trials.
UK data present an early insight into the real-world effectiveness of COVID-19 vaccines and the remaining challenges post-vaccination. A previous analysis of community-based individuals in the COVID Symptom Study showed a significant reduction in infection post-vaccination from 12 days after the first dose, findings that were recapitulated in a UK-based, real-world, case-control study.
National surveillance data from the first 4 months of Israel's vaccination campaign showed that two doses of BNT162b2 reduced both symptomatic and asymptomatic infections, COVID-19-related hospitalizations, severe disease and death.
Nonetheless, some people still contract COVID-19 after vaccination, and further virus variants could evolve with increased transmissibility (as with B.1.1.7 [the alpha variant]). Indeed, variants of concern have shown reduced neutralization by convalescent and post-vaccination serum samples in vitro, and led to increased rates of post-vaccination infection compared with the original outbreak variant in early findings from a real-world case-control study.
Vaccination (compared with no vaccination) was associated with reduced odds of hospitalization or having more than five symptoms in the first week of illness following the first or second dose, and long-duration (≥28 days) symptoms following the second dose.
Almost all symptoms were reported less frequently in infected vaccinated individuals than in infected unvaccinated individuals, and vaccinated participants were more likely to be completely asymptomatic, especially if they were 60 years or older.