STOP-CoV study results highlights importance of completing primary vaccine series

This is a summary, written by members of the CITF Secretariat, of:

In a pre-print, not yet peer reviewed, the team from the STOP-CoV study, led by Dr. Sharon Walmsley from the University Health Network in Toronto, described their most recent findings: younger adults (30-50) generate higher levels of vaccine-specific antibodies compared to older adults (70 and above) after both the first and second mRNA vaccine doses. They also found that antibody levels after the second dose were generally higher for people who received two doses of Moderna or a Moderna/Pfizer mix compared to those who received two doses of Pfizer or other vaccine combinations, irrespective of age. Regarding reactions to vaccines, their data show that younger adults are more likely to experience temporary minor adverse events following immunization than the older cohort.

The paper addresses important issues related to vaccine safety and antibody response to vaccination.

Key points:

• Vaccine safety:
  • Among all participants who completed the survey, the most common side effects in the week after vaccination were temporary pain at the injection site (70%), fatigue (60%), and generally feeling unwell (46%).
  • Younger participants were more likely to experience minor adverse events following immunization compared to older participants.
  • Adverse events after the second dose were more likely for those who received the Moderna vaccine (irrespective of first dose) compared to those who received Pfizer-BioNTech.
  • 1% of respondents self-reported persistent adverse events thought to be associated to the vaccine five months after the injection.

• Vaccine antibody response:
  • 46% of the older participants were positive for spike receptor-binding domain (RBD) antibodies after the first dose, which climbed to 98% after the second dose, emphasizing the importance of completing the primary vaccine series. At 12 weeks following the second dose, RBD antibody positivity remained at 96%.
  • RBD antibody positivity for younger participants rose from 84% after the first dose to 100% after the second. This percentage remained stable at 12 weeks post-second dose.
  • RBD antibodies after the second dose were generally higher for individuals who received two doses of Moderna or a Moderna/Pfizer mix compared to those who received two doses of Pfizer or other vaccine combinations, irrespective of age.
  • A longer interval between doses was associated with lower levels of RBD antibodies prior to the second dose. However, there were no significant differences in antibody levels at 12 weeks post-second dose, irrespective of how long there had been between the first two doses.

Participants were invited to submit blood samples via dried blood spot kits one week after the first dose, three weeks after the first dose, two weeks after the second dose, and 12 weeks (~three months) after the second dose.

Learn more about the STOP-CoV study here.