Thought of the Day - June 7 2021 - Which Vaccine to Choose?

With Canada shortening the wait time for a second vaccine dose, most Provinces are now inviting citizens to book their second shot. With this second invite, the option to select which brand to receive, subject to availability, has been put on the table in some Provinces. The main driver for this change in policy is that by delaying the second shot here in Canada the same brand may not be available for all in a timely manner.

So what information is available concerning the risk/benefit of mixing vaccines?

The only paper I can find published in Canada is from the National Advisory Committee on Immunization published on June 1 which is requoting some early results from Europe (UK, Spain and Germany) that are studying the effect of mixing AstraZeneca and Pfizer. These seem to imply by mixing there may be a higher immune response with some additional associated side-effects but none serious.

Indeed the average effectiveness of mixing brands should be higher than two shots of the same. The logic is this. Due to the complexities of the immunity systems, some people will have lower rates with a particular brand. Therefore, switching brands will significantly increase the immunity of these specific people (some for example from almost 0% to 65%). These people will see a much bigger increase than the average boost from the second shot (for example from 65% to 90%). Thus a population with mixed shots will end up having a higher average immunity than a homogeneous population.

But this is only half the story. What about side effects? Logically the more types of vaccine you try the more likely you are to find one to which you have a negative reaction. So if you did not have significant issues with your first shot, but you did get some reaction (suggesting the vaccine did do something), you may want to stay with it.

There is now more information published on vaccine side effects for various demographics. For example, Adenoviral vaccines (AstraZeneca/Covishield and Johnson & Johnson) are a higher risk for young women, while mRNA (Pfizer and Moderna) present more risk for young men. So if you have specific criteria in mind you may want to do some research.

As there has been almost no use of J&J here in Canada, it is unlikely to be offered as a second shot. Note, there is no fundamental reason why J&J is a single-dose vaccine. J&J just got the dosing right in their clinical trials, and thus reported very good numbers. Moreover, as they were late to the party, it was decided to launch as a single dose vaccine to avoid the time needed to complete second dose trials.

So in conclusion, it seems logical that mixed vaccines should be more effective on average, but with a slightly higher risk of adverse reaction. At least with the eight-week wait, your immune system should be back to fighting strength ready to do it again.

So which will people choose?

Most will simply choose is to stick with the same brand as they received the first time. Pfizer has been the most dominant brand here in Canada. 70% of the shots administered to date in Canada are Pfizer, and it is still widely available. I predict that Pfizer will be even more popular for the second shot as:

• Most people that had Pfizer for their first shot will see no reason to switch;

• Moderna is in short supply and may not be available to the 20% of Canadians who had it for their first shot, so they may be pushed to Pfizer;

• Bad publicity (discussed in several Vaccine Wars posts) will continue to drive people away from AstraZeneca (the latest news being Health Canada changing AstraZeneca's expiry date);

• J&J is a non-starter in Canada;

• Oh, and remember your tax money has already been committed to a couple more years of Pfizer booster shots, so you may as well get used to it.

Finally, as seen in the US, and with Canada still not having clear recommendations including on vaccine passports, fewer people will actually choose to get a second shot.

Happy dosing.

Cheers

Cliff