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Wednesday, November 30, 2022
Letter: What happens if you have a different vaccine for second dose?


Dear editor:

On May 8, the Ontario chief medical officer of health Dr. David Williams stated that the AstraZeneca (AZ) vaccine would no longer be used as a first dose in Ontario. 

Since the first use of the AZ vaccine, in Ontario the case rate for vaccine-induced immune thrombocytopenia (VITT), a rare blood clot, had increased from 1.0 to 1.7 per 100,000 persons. In the U.K. the case rate was 1 in 93,000, whereas in Canada the case rate was 1 in 60,000.

On May 20, Williams announced that the 1 million persons in Ontario who received the AZ vaccine as their first jab, could select AZ as their second jab if they so desired.

So how are you supposed to decide what to do?

First the good news: Your body uses two main mechanisms to fight against COVID-19 – antibodies and killer T cells.

According to Dr. Marc Hellerstein from University of California Berkeley, a strong antibody response correlates with more severe disease in COVID-19 cases, whereas a strong T cell response is correlated with less severe disease. 

So T cells being elevated is a good thing. They are the cells that will kill COVID-19 if you get infected. The antibodies are there to act as helpers.

A study by the University of Birmingham in the U.K. published in April looked at patients over 80 years age who had received the AZ vaccine. After five weeks of study, 87 per cent of them had raised levels of antibodies against COVID-19 and Pfizer vaccinated patients had 93 per cent raised antibodies. 

When it came to T cells, 31 per cent who got the AZ vaccine had an increase, whereas only 12 per cent of those who got Pfizer showed an increase. The researchers speculated that at 12 weeks the levels would be closer. So, if you got the AZ as a first jab, it appears you have better short-term protection against COVID-19.

Now the somewhat good news: On May 12, the U.K. Com-COV study results were published in the Lancet medical journal. The participants were randomized to receive an AZ vaccine, followed by an mRNA vaccine such as Pfizer. Or vice versa. The study looked at side effects after 28 days and found there were more adverse events if two different vaccines were received. 

For example starting with an AZ vaccine and receiving a Pfizer vaccine as a booster caused fever in 34 per cent of cases as opposed to 11 per cent if both doses were AZ. The same results were found for chills, fatigue, headache, joint pain, malaise and muscle aches. 

Most of the adverse events occurred within the first 48 hours after the injection and lasted a few days. Patients were able to treat their symptoms with over-the-counter medicines such as acetaminophen (Tylenol) or ibuprofen (Advil, Motrin). The study is on-going to evaluate whether the immunogenicity of a mixed-dosing regimen produces the same results in terms of fighting COVID-19.

So what's the bottom line? 

Whether you got the AZ or Pfizer vaccine, you are protected against COVID-19. Once you receive your second jab you have better protection. Those who got AZ as a first shot and take AZ as their second shot will have fewer side effects and better protection against the variants. Those who got AZ as their first shot and take Pfizer as a second shot will have a few more side effects for 48 hours.

The best news, however, is that the case rate of blood clots with the AZ vaccine went from 1 in 60,000 for the first jab to 1 in 600,000 after the second jab. And now they know how to treat it.

I hope this helps you make your decision.

Robin Jinchereau