MEMO: Optimal Interval Between First and Second Doses of COVID-19 Vaccine   

TO:  Pharmacy and Health Care Professionals in Niagara 

FROM:  Pandemic Response Division, Niagara Region Public Health

REFERENCE: Optimal Interval Between First and Second Doses of COVID-19 Vaccine  

DATE:  Nov. 12, 2021

Since COVID-19 vaccination began, studies have been evaluating the impact of longer intervals used by countries such as Canada and the United Kingdom on protection against COVID-19.

Research shows that the very good protection provided by COVID-19 vaccines is further improved when the interval between the first and second doses is extended beyond the authorized intervals. Specifically, extending the interval leads to (a) greater protection after the second dose, and (b) greater durability of protection such that immunity does not wane.

According to National Advisory Committee on Immunization (NACI), the optimal interval between the first and second dose of a two-dose COVID-19 vaccine series is

• 8 weeks for the mRNA COVID-19 vaccines
• At least 8 weeks for the AstraZeneca Vaxzevria COVID-19 vaccine

Actions for Pharmacy and Health Care Professionals

• If you are vaccinating a patient with a first dose of COVID-19 vaccine or are providing information on the dosing interval, recommend NACI’s optimal interval. According to NACI, when recommending a longer dose interval, local transmission of the SARS-CoV-2 virus; the degree of individual risk of exposure to the virus; and the need of a second dose for earlier protection should be considered and be balanced against the benefit of stronger and longer-lasting protection, but commencing later.
• Individuals at higher risk for severe outcomes or exposure to COVID-19 include
  • Adults 60 years old or over
  • Individuals with underlying medical conditions
  • Pregnant individuals
  • Residents and staff of congregate living settings
  • Adults in Indigenous communities
  • Adults in racialized and marginalized communities
  • Health care workers and first responders
  • Frontline essential workers who cannot work virtually
• Remind all patients to continue to follow infection prevention measures. The first dose provides some protection, but the second dose is needed for full protection. Patients are considered fully vaccinated 14 days after receiving their second dose.
• Reassure patients who may be concerned that they received their second dose at an interval different than the current optimal interval stated by NACI that they still have very good protection against COVID-19 and do not need to restart their vaccine series

More Information

Call the COVID-19 Primary Care Info-Line at 905-688-8248 (press 7, then 1 for physicians). The info-line is open Monday to Friday, 8:30 a.m. to 4:15 p.m.