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  • A UK study published in Lancet (not yet peer-reviewed) looked at symptomatic and asymptomatic infection after a single dose of Pfizer vaccine in a cohort of 23,000 healthcare workers (HCWs).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

Overall, the research is beginning to emerge on potential relationship of cerebral venous sinus thrombosis (CVST) with thrombocytopenia and adenoviral vector COVID-19 vaccine. A case series of 12 U.S. patients who received J&J/Janssen vaccine and developed CVST and thrombocytopenia as reported to VAERS between March 2 and April 21, 2021 and published on April 30, 2021 looked at the characteristics of these patients. It found following with respect to the patient characteristics who received this type of vaccine and developed subsequent CVST with thrombocytopenia:

  • Seven patients had at least 1 CVST risk factor, including obesity (n = 6), hypothyroidism (n = 1), and oral contraceptive use (n = 1); none had documented prior heparin exposure.
  • Time from vaccination to symptom onset ranged from 6 to 15 days. Eleven patients initially presented with headache; 1 patient initially presented with back pain and later developed headache. Of the 12 patients with CVST, 7 also had intracerebral hemorrhage; 8 had non-CVST thromboses.
  • 11 patients tested for the heparin-platelet factor 4 HIT antibody had positive results.
  • 10 patients were hospitalized in ICU. Outcomes were death (n = 3), continued ICU care (n = 3), continued non-ICU hospitalization (n = 2), and discharged home (n = 4).



Updated: May 5, 2021