Reducing the time interval for second dose after first AstraZeneca dose


One question not addressed in the article above by Dr. Alexander Wong is the dosing interval between the first AstraZeneca (AZ) vaccine, and the second dose (whether AZ or mRNA). This is a topical question, which I address in the commentary that follows.

On May 28, the Ontario government announced that it was accelerating the rollout of second doses, but it was keeping the earlier established 12-week required interval between the first and second dose for persons who got AZ. On June 3 the announcement came that those who received a first dose of AZ can choose to either receive a second dose of AZ, or an mRNA (Pfizer or Moderna) vaccine for their second one. However, the dosing interval was kept at 12 weeks, no matter which choice the person made regarding the second dose.

Maintaining such a long dosing interval after a first dose of AZ has raised criticism, including from us at RNAO. Such guidance contrasts sharply with the shortened interval for those who received a first dose of mRNA, creating what appears to be a substantial inequity between the two groups. But beyond the apparent inequity, there is a real concern with such a delay.

The National Advisory Committee on Immunization (NACI) has recommended the interchangeability of AZ or mRNA second dose following the first AZ dose. They cite a UK study where an mRNA vaccine was given 4 weeks after the initial dose, a similar study in Spain that adopted an 8-week dosing interval, and one in Germany that adopted a 12-week interval. NACI summarizes: “Emerging evidence indicates that mixed COVID-19 schedules have an acceptable safety profile, which has been demonstrated at multiple dosing intervals between 4 and 12 weeks.”

Although the daily case numbers are gradually dropping in Ontario, the Delta variant (B.1.617, first discovered in India) is taking hold and becoming the dominant one, particularly in hotspots such as Peel, Toronto, York, Halton and Waterloo. According to the Science Advisory Table the Delta variant is about 50% more transmissible than Alpha (B.1.1.7, first discovered in the UK) and may be more dangerous, with the risk of hospitalization increased. It will likely be the dominant form of the virus in Ontario (and Canada), this summer. The scientists caution that it is critical to control the spread of this variant.

According to the same advisory body, having only one dose of any COVID vaccine is less effective against symptomatic disease for the Delta variant than for the Alpha variant. The first dose may still protect against severe disease, so its importance should not be diminished. However, the second vaccine dose is more than twice as effective against the Delta variant as just having one dose. That is the reason they urge us to accelerate getting second doses in arms.

On June 10th the Ontario government announced it was accelerating even further second doses in Delta hotspots where the variant is becoming dominant. However, it surprised many of us when it excluded once again those who received AZ.

The Twittersphere took on, urging the government to speed up the dosing intervals for persons who received the AZ vaccine. Reacting to the June 10 announcement, I said “RNAO is disappointed Minister Elliott didn’t announce interval change between AZ and Pfizer or Moderna. The time is NOW - Delta is catching up!!! MOST experts support 4-week interval!” Please continue to RT to encourage change.

RNAO continues to pressure for change to the AZ dose interval in various media outlets. QP Briefing quoted our tweet; you can RT here

Today, I was amongst the fortunate to get my second dose! In tweeting a selfie, I took the opportunity to push once again to shorten the dosing interval following AZ. Here’s for you to RT.

There is no doubt that Ontario MUST shorten the dosing interval to receive an mRNA vaccine following a first dose of AZ. The question is – how long will we have to wait for that decision, and why do we need to wait at all? Each day that passes is a missed opportunity to gain more protection against a dangerous variant.

IMPORTANT UPDATE: Just as I was finishing this commentary, the announcement reached my inbox that the Ontario government is reducing the required interval to 8 weeks from 12 weeks. Once again, the Twittersphere proved its power, even on a Saturday!

The announcement says: “With informed consent, individuals can choose between a second dose of AstraZeneca or an mRNA vaccine, at an eight to 12-week interval, recognizing that while waiting 12 weeks helps to ultimately provide more protection, some may choose to receive their second dose sooner to have the increased protection provided by a second dose earlier. All of these options provide protection against COVID-19, including the Delta variant, and have been deemed safe.”