Your COVID-19 vaccine questions answered

More than 10 per cent of Hamilton's eligible population has had at least one dose of a COVID vaccine.

What difference will vaccination make? What vaccine is best? And does waiting four months for the second dose matter?

Two prominent McMaster University experts answer your questions - Dr. Dominik Mertz, associate professor in the division of infectious diseases, and Dr. Mark Loeb, Michael G. DeGroote Chair in Infectious Diseases. Answers have been edited.

1) Does what vaccine you get make a difference?

Mertz: Real-world data will show how these vaccines really compare, but all approved vaccines seem to be highly effective in preventing severe infections."

Loeb: The mRNA COVID-19 vaccines authorized in Canada - Pfizer-BioNTech and Moderna - after two doses have about 94 per cent efficacy. The nonreplicating viral vector vaccine authorized in Canada - AstraZeneca - has 62 per cent efficacy in those 18 to 64. These are short-term results and clinical trials are ongoing."

2) How effective is the first dose of vaccine?

Mertz: Immunity needs time to build. Based on the limited data we have, protection is quite good already two-plus weeks after a first dose, but seems to remain lower than one to two weeks after the second dose - difference is probably in the 10 to 20 per cent range."

Loeb: It is lower than the efficacy after the second dose for mRNA COVID-19 vaccines (Pfizer and Moderna). The protection offered by the viral vector vaccine (AstraZeneca) after one dose is comparable to that after two doses."

3) Am I protected right away after I get the first dose?

Mertz: It takes at least two to three weeks to build optimal immune response from the first vaccine."

Loeb: Not right away as it takes time for immunity to build up. It takes 14 days for the protection to build up to the highest levels."

4) How quickly does the protection wane?

Mertz: We do not know yet, the vaccines have not been around long enough."

Loeb: This is not yet known. Studies are ongoing."

5) What changes in terms of efficacy when a second dose is delayed to four months?

Mertz: We do not have evidence that would suggest that delaying the second dose will reduce final efficacy, but we will learn more from the real-world data that is being collected."

Loeb: There are no clinical trial data to address changes in efficacy.

7) Can a vaccinated person still transmit COVID? And what's the likelihood of that happening?

Mertz: Yes, it is not 100 per cent protection from neither mild disease nor asymptomatic disease. Data suggests that protection from asymptomatic infection and as such risk of transmission may be reduced to as much as 90 per cent, but may well be lower than that. Still lots of uncertainty, and real-world data will get us better estimates in the near future."

Loeb: This is unknown but it is a possibility."

8) If I'm vaccinated, do I have to still follow public health measures?

Mertz: The vast majority of people are not immune, yet, and because it does not protect 100 per cent from carrying the virus, vaccinated people can still serve as a source."

Loeb: Yes."

9) If I am vaccinated, can I loosen restrictions when I'm around other vaccinated people such as hanging out indoors without masks, sharing a meal, hugging etc.?

Mertz: Not according to current public health rules, but people may choose to do so knowing that risk is not zero per cent, but hugely decreased to a level where individuals can feel comfortable doing so. The United States Centers for Disease Control and Prevention guidelines would suggest that it is OK to do so among fully vaccinated individuals."

Loeb: Best to continue precautions."

10) As the vaccination rate slowly climbs, when can the broader population start to relax when it comes to public health restrictions?

Mertz: This will - and has to - eventually happen, but still several months away as we first need to build enough immunity in the population. Israel is currently the poster child in terms of vaccine rollout and how this will eventually influence public health measures, and we can watch closely how this will work out for them, and learn."

Loeb: It will depend on what is happening with respect to transmission."

11) Is there a number for herd immunity?

Mertz: Herd immunity is typically quoted to have been in the range of 70 per cent but this would be for sterilizing immunity - 70 per cent-plus must no longer be susceptible for the virus at all. We will first achieve immunity that largely prevents disease and will as such reduce the impact of the virus significantly, before we may be able to achieve herd immunity at the population level - if ever. But if we can largely prevent severe infection, we achieved our main goal."

Loeb: There are figures at least in theory but none have been established using collected data."

12) Why is Johnson and Johnson just one dose?

Mertz: The vaccine does not require a booster dose to achieve a sufficiently strong immune response."

Loeb: Early phase studies suggested that one dose would work and so this vaccine was tested in clinical trials with just one dose."

13) I have heard that AstraZeneca is associated with blood clots? Is that true and should I avoid this vaccine?

Mertz: The vaccine continues to be considered safe and the potential risk of blood clots is negligible compared to the benefit from the vaccine. Blood clots do not seem to be more frequent in the vaccinated population than they would be expected by chance alone, although, there is a mechanism that might be vaccine associated that may have triggered a subgroup of these blood clots observed."

Loeb: Benefits outweigh possible risks. But evidence is currently not sufficient to conclude with certainty whether these are caused by the vaccine or not."

Joanna Frketich is a Hamilton-based reporter covering health for The Spectator. Reach her via email: jfrketich@thespec.com