What you need to know about getting a second COVID shot
COVID vaccination is more important than ever as Ontario is in a race with the fast-spreading Delta variant.
The Delta strain will become the predominant strain in Ontario," said associate chief medical officer of health Dr. Barbara Yaffe. If you're vaccinated - particularly if you've had two doses - you are very well protected ... The more people who are protected the less the strain can spread."
There is concern the variant, first found in India and also known as B.1.617.2, causes more severe illness.
A lot of our hospitalizations are people who have been unvaccinated," said Dr. David Williams, Ontario's chief medical officer of health. There's an even greater need for you to get vaccinated with the variant Delta moving around."
Do I need to get my second dose early?
Yes, early research shows one dose is only about 33 per cent effective against the Delta variant of concern (VOC). It takes two to five weeks after the shot to be fully protected.
The sooner you get your second dose, the earlier you will benefit from maximal protection from the vaccine - even more important in light of the Delta VOC where the gain in protection from first to second dose is even larger than with previous variants," said Dr. Dominik Mertz, associate professor in the division of infectious diseases at McMaster University.
In addition, vaccine rates - including second doses - determine when Ontario moves to a new step of its reopening plan.
Is it safe to get a different vaccine for my second dose?
Based on three studies reported to date, mixing doses appears to be safe," said Dr. Mark Loeb, the Michael G. DeGroote Chair in Infectious Diseases at McMaster. One of the three studies reported more minor side effects like pain at the injection site but this was short-lived."
A decision aid developed by Canadian doctors, pharmacists, scientists and patients - including Dr. Menaka Pai from McMaster - points out, We have used different vaccines for boosters in the past for other viruses like hepatitis A, influenza and polio. It is safe and is not linked with any long-term side effects."
Do I get better protection from mixing doses?
The data is not yet that robust," said Mertz. We do not have head-to-head comparison with clinical outcomes."
As a result it's unknown if the protection is less, the same or more when doses are mixed.
There are no studies yet that have reported what happens with actual vaccine protection when you mix doses," said Loeb.
What if I got AstraZeneca first?
Getting two does of AstraZeneca is very effective at preventing serious illness from the original COVID-19 and the early variants," states the decision aid.
Getting Moderna or Pfizer as the second dose instead should be at least as protective," it predicts.
Early studies show that people generate a very good immune response ... when Moderna or Pfizer are used after AstraZeneca," it states.
What if I got Moderna first?
It is best to have the same mRNA vaccine as your first dose," said Loeb.
As a result, the province recommends getting Moderna as your second shot unless it is unavailable, in which case get Pfizer.
Moderna and Pfizer are essentially the same vaccine," states the decision aid.
What is the best protection against Delta?
Two doses of Moderna or Pfizer appear to offer more protection against symptomatic infection from Delta than two doses of AstraZeneca," states the decision aid. If you had AstraZeneca for dose one, we don't know yet if getting Moderna or Pfizer for dose two will give you better protection from Delta or future variants."
What about VITT?
AstraZeneca has been linked with a very rare but serious blood clotting disease called vaccine induced thrombotic thrombocytopenia, or VITT," states the decision aid. VITT can be fatal. According to reports from several countries including Canada, there are around 17 cases of VITT for every million people given dose one of AstraZeneca. For dose two, the risk of VITT is estimated to be two for every million vaccinated. These estimates may change as more people around the world have dose two and we learn more. The Moderna and Pfizer vaccines do not cause VITT."
Put another way, Mertz and Loeb both say the risk is one in 600,000 for a second dose of AstraZeneca based on data from the United Kingdom.
Will I need a third dose?
As of now, two doses of any vaccine is considered fully immunized," says the decision aid.
However, that could change if boosters are found to be needed in the future.
The need for a third dose is to be determined for any of the vaccines, regardless of whether vaccines have been mixed or not," said Mertz.
Joanna Frketich is a Hamilton-based reporter covering health for The Spectator. Reach her via email: jfrketich@thespec.com