A recent study conducted in Germany and published on 1 June as a preliminary document without peer review (“preprint without peer review”) provides additional information on the mixture and association between AstraZeneca and Pfizer vaccines. These preliminary data relate to 26 people, ages 25 to 46, who received the first dose of AstraZeneca vaccine, followed by a second dose of Pfizer eight weeks later.
Neutralizing activity was 3.9 times greater for the alpha variant (B.1.1.7) and similar to the delta variant (B.1.617.2) compared to the neutralizing activity observed in subjects who received two doses of Pfizer. No health problems were observed.
Finally, a small Canadian study was conducted at Dalhousie University in 66-year-old volunteers who were given a first dose of AstraZeneca vaccine followed by a second dose of Pfizer vaccine 33 days later. It has been reported that immune responses are robust without any health concerns.
Risk of TTIV with a second dose of AstraZeneca
The risk of developing TTIV with a second dose of AstraZeneca for people who received the first dose of this vaccine is very low. The best data currently available is from the UK watchdog. As of May 27, 17 cases of TTIV have been reported after receiving 10.7 million second doses of the AstraZeneca vaccine, a risk of approximately 1 in 600,000.
So what is the best option?
The current and planned availability of the two mRNA vaccines in Canada is excellent, with steady supplies expected through June and July. This means that in most cases it will not be necessary to wait for the preferred option.
I was fortunate enough to receive two doses of the COVID-19 vaccine in early 2021, so I don’t have to make a decision for myself. However, many people have asked me for advice on behalf of their loved ones, friends and themselves.
Although the data aren’t definitive, evidence is mounting for Pfizer’s mix-and-combination approach to AstraZeneca, which is at least as good (if not better) than two doses of the same vaccine. There are no inherent risks in mixing vaccines, and no health problems have been noted so far.
In addition, taking the mRNA vaccine completely avoids the risk of TTIV. Although this risk is very low, TTIV is dangerous and may be fatal.
For these reasons, I believe that if a second dose of the mRNA vaccine (Pfizer or Moderna) can be obtained, it is best for most people who have received a first dose of AstraZeneca.
AstraZeneca case
There are many reasons why a person would choose AstraZeneca over the mRNA vaccine for their second dose. There are no data on the clinical efficacy of the vaccine combination. For this reason, some people may prefer a “proven” method of receiving two doses of AstraZeneca. Others, who did not experience any side effects with their first dose of AstraZeneca, may choose a second dose of the same vaccine to avoid side effects.
The UK COM-CoV study will provide data on immunity (antibody response) later this month. This data may or may not support the blending approach. Some may prefer to wait for this data before making a decision. Others will take the vaccine given to them first.
Whatever decision one makes, the bottom line is that everyone gets a second dose as soon as they qualify, whether it’s the AstraZeneca vaccine or the mRNA vaccine. Available data supports the assertion that both options are safe and effective, so there is no “wrong” option. Full vaccination provides optimal protection against current and emerging strains, including the delta variant.
In Canada, we are fortunate to be able to choose between two great options for our second dose. Nous avons la responsabilité de veiller à ce que les stocks de vaccins non utilisés ne soient pas gaspillés, et nous devons faire beaucoup plus pour soutenir l’équité mondiale en matière de vaccins afin de la contribufiner COVID-19 the world.
Please go vaccinate yourself and your community!