What new data has emerged since the clinical trials?

We are beginning to get population-level data as vaccines are rolling out worldwide.

  • Recent population-level data from Israel show that among 600,000 vaccinated individuals, there was a 94% drop in symptomatic COVID-19 and participants were 92% less likely to develop severe disease. The comparison was against a group of the same size, with matching medical histories, who had not received the vaccine.
  • Another study from Israel released in the Lancet, looked at 9,109 vaccinated and unvaccinated health care workers. They found:
    • Adjusted rate reductions for all COVID-19 infections (symptomatic and asymptomatic) were 30% for days 1-14 and 75% for days 15–28 after the first dose.
    • Adjusted rate reductions for symptomatic COVID-19 infections were 47% for days 1-14 and 85% for days 15–28 after the first dose.
  • A UK study published in Lancet (not yet peer-reviewed) looked at symptomatic and asymptomatic infection after a single dose of Pfizer vaccine in a cohort of 23,000 healthcare workers (HCWs). The regular PCR-testing of participants, regardless of symptom status, allowed for the detection of asymptomatic infection.
    • Vaccine effectiveness against infection for the vaccine was estimated to be at least 70% 21 days after the first dose, increasing to at least 85% 7 days after the second dose in the study population.
  • Another study from the UK tested healthcare workers after the Pfizer vaccine compared to their non-vaccinated peers. This group did not include and HCW with the second dose.
    • Twelve days post-vaccine, 4/1989 (0.37%) tested positive for COVID compared to 26/3252 (0.8%) of unvaccinated HCW over the same time period. Over a 6-week period following vaccine, 4/4872 (0.10%) vaccinated HCW tested positive, compared to 113/14083 (0.80%) unvaccinated.
  • In the United States, new data released by the CDC of 3,950 health care personnel, first responders, and other essential and frontline workers in eight locations found that under real-world conditions, mRNA vaccine effectiveness of full immunization (≥14 days after second dose) was 90% against SARS-CoV-2 infections regardless of symptom status; vaccine effectiveness of partial immunization (≥14 days after first dose but before second dose) was 80%.
  • In the United States, non-peer-reviewed data has emerged from the UCLA and UCSD health systems which have either mandated or encouraged asymptomatic testing in health care workers.
    • From December 16, 2020, through February 9, 2021, a total of 36,659 health care workers received the first dose of vaccine, and 28,184 of these persons (77%) received the second dose.
    • Among the vaccinated health care workers, 379 unique persons tested positive for SARS-CoV-2 at least 1 day after vaccination, and the majority (71%) of these persons tested positive within the first 2 weeks after the first dose.
    • After receiving both vaccinations, 37 health care workers tested positive; of these workers, 22 had positive test results 1 to 7 days after the second dose.
    • Only 8 health care workers tested positive 8 to 14 days after the second vaccination, and 7 tested positive 15 or more days after the second vaccination
    • In this cohort, the absolute risk of testing positive for SARS-CoV-2 after vaccination was 1.19% among health care workers at UCSD and 0.97% among those at UCLA



Lancet HCW

HCW Study

CDC Study

California Health System Data


Updated: March 31, 2021