BACKGROUND AND PURPOSE:

• Thompson et al. (NEJM, 2021) compared infection rates, mean viral RNA loads, and symptoms and the duration of illness among partially vaccinated, fully vaccinated, and non-vaccinated individuals

METHODS:

• Prospective cohort study
• Participants
  • Health care personnel, first responders, and other essential and frontline workers
• Exposures
  • mRNA-based COVID-19 vaccination status
• Study design 
  • Participants completed weekly SARS-CoV-2 testing (between December 14, 2020, to April 10, 2021)
  • The formula for calculating vaccine effectiveness was 100%×(1−hazard ratio for SARS-CoV-2 infection in vaccinated vs. unvaccinated participants)
  • Adjustments were made for the propensity to be vaccinated, study site, occupation, and local viral circulation
• Primary outcome
  • Time to RT-PCR–confirmed SARS-CoV-2 infection in vaccinated participants vs unvaccinated participants
• Secondary outcomes
  • Viral RNA load
  • Frequency of febrile symptoms and duration of illness among participants with SARS-CoV-2 infection

RESULTS:

• 3975 individuals
• SARS-CoV-2 positive tests: 5% (204 participants)
  • Fully vaccinated (≥14 days after dose 2) participants: 5 cases
  • Partially vaccinated (≥14 days after dose 1 and <14 days after dose 2) participants: 11 cases
  • Unvaccinated participants: 156 cases
• Adjusted vaccine effectiveness
  • Full vaccination: 91% (95% CI, 76 to 97)
  • Partial vaccination: and 81% (95% CI, 64 to 90)
• Mean viral RNA load was significantly lower in partially or fully vaccinated participants with breakthrough infection vs unvaccinated
  • RNA Load: 40% lower (95% CI, 16 to 57)
• The risk of febrile symptoms and duration of illness were also lower in the breakthrough vaccine group
  • Febrile illness: Relative risk 0.42 (95% CI, 0.18 to 0.98)
  • Duration of illness: 2.3 fewer days spent sick in bed (95% CI 0.8 to 3.7)

CONCLUSION:

• mRNA-based COVID-19 vaccines are highly effective at preventing infection in real-world conditions
  • 91% with full vaccination
  • 81% with partial vaccination
• Among individuals who experienced breakthrough infection, vaccination also reduced the viral RNA load, risk of febrile symptoms, and duration of illness

The authors state

If further data confirm that the administration of mRNA vaccines reduces the number of viral RNA particles and the duration of viral RNA detection, thereby blunting the infectivity of SARS-CoV-2, then the overall results support that mRNA vaccines not only are highly effective in preventing SARS-CoV-2 infection but also may mitigate the effects of breakthrough infections — a finding that is especially important to essential and frontline workers, given their potential to transmit the virus through frequent close contact with patients, coworkers, and the public

Learn More – Primary Sources:

Prevention and Attenuation of Covid-19 with the BNT162b2 and mRNA-1273 Vaccines

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