Thousands of Ontarians found themselves unexpectedly in immunization limbo Tuesday afternoon, following an announcement that the province was suspending the use of AstraZeneca following an uptick in rare blood clotting complications.

Thousands of Ontarians found themselves unexpectedly in immunization limbo Tuesday afternoon, following an announcement that the province was suspending the use of AstraZeneca following an uptick in rare blood clotting complications.

More than 850,000 doses of the beleaguered British vaccine have been administered in the province so far, many of them to jubilant Gen Xers who turned out in droves after the shot’s age of eligibility was dropped to 40 last month.

Many of those people are wondering what this announcement means for them, particularly with the time for a second dose rapidly approaching. So what happens now?

The short answer from officials is that this change was made out of an “abundance of caution” and that the risks remain low relative to the risk of COVID-19. Dr. David Williams, Ontario’s chief medical officer, says they’re watching research on options like mixing and matching doses closely, and will have more guidance soon.

Still, constantly changing advice on the vaccine has left much confusion in its wake. If you’ve had one dose and are wondering what happens next, here’s what you need to know.

Should I not have taken the AstraZeneca vaccine?

As Williams put it, everyone who took AstraZeneca in recent weeks “did absolutely the right thing to prevent illness, and to protect their families, loved ones, and communities.”

Having a first dose of any vaccine provides significant protection not only against COVID, but against serious illness and death should you become infected.

The need to get vaccinated was particularly urgent given the increase in cases in Ontario, as well as the burden that had been placed on hospitals by a growing number of severely ill patients.

Still, you said we should take the first vaccine offered. Now there’s a problem? What changed?

It is not that the vaccine has suddenly been shown to be hugely risky, Williams told media Tuesday, but there has been a slight uptick in reports of a rare, but serious, blood clot that has been linked to the vaccine.

Up until now, about one blood clot has been reported for every 100,000 AstraZeneca doses given out. But in recent days, that number has changed to one in about 60,000, according to Dr. Jessica Hopkins of Public Health Ontario.

Hopkins stresses that this still counts as a very rare risk. Clots are still more likely if you have COVID-19.

But because of the increase, which was flagged by the monitoring system put in place to track side effects from the vaccines, experts want to take a little time to investigate what is going on.

What is the concern here with AstraZeneca?

The rare blood clot that has been linked to the AstraZeneca vaccine is called a vaccine-induced thrombotic thrombocytopenia, or VITT for short. There have been 12 cases in Canada so far linked to the more than 2,300,000 shots distributed in Canada. Three people have died.

While doctors say it can be treated if caught early, it is still serious and can be fatal.

Anyone who has had a dose in the last month should still continue to monitor for a wide range of symptoms, including headache, blurred vision, shortness of breath or significant changes in a limb, including swelling, redness or a cold feeling.

Aren’t I still more likely to get seriously ill from COVID?

It’s possible to think about risk as an individual, or as part of a community, Hopkins says.

There is a spectrum of illness that can be caused by VITT, from being mildly symptomatic to seriously ill. COVID can also really range but depends a lot on your personal circumstances. Are you an essential worker? Can you work from home? There are factors to consider when thinking about your own risk from COVID.

But public health officials are looking at this from a population level — “overall, does this safety signal give us enough concern that we would preferentially choose other products?”

Right now, as cases drop, incidents of VITT rise and other vaccines become available, it makes sense to pause it, she says.

But that doesn’t mean that an individual shouldn’t still choose the AstraZeneca vaccine for themselves, based on their personal risk calculation.

What about my second dose?

Public health officials are expected to release guidance on options for second doses in the near future, but are looking at a couple of options.

According to data from the United Kingdom, where AstraZeneca has been used extensively, the vast majority of people who get blood clots get them after the first dose. The risk of a clot after a second dose could be as low as one in a million.

Hopkins said they’d like to get more information about the safety of second doses before making a recommendation.

If the risk of a clot after a second dose of AstraZeneca is so low, why can’t I have one now?

Experts are reviewing the UK data before the government makes a recommendation, Williams said.

“It’s one of the real options we’re looking at to make sure that if we are going to recommend people do the second dose with the AstraZeneca vaccine, they have all the information they need to make an informed consent on that matter.”

Can I have a different vaccine for my second dose?

Maybe, but we don’t know yet.

There are also trials ongoing — including a major one at Oxford University — that is studying whether or not you can mix and match vaccines. Meaning, give someone a dose of a viral vector vaccine like AstraZeneca, but then follow it up with an mRNA, like Pfizer or Moderna.

Canadian officials have previously said they’re watching that study closely, as being able to use vaccines interchangeably would ease distribution challenges, particularly given the reliable supply of Pfizer vaccines. Results from that trial are expected soon.

Williams said that the National Advisory Committee on Immunization has been asked to provide guidance on whether mixing and matching vaccines is recommended.

Does this mean I’m going to have to wait longer for that second dose?

Williams said the government is still aiming to have second doses distributed between 12 and 16 weeks after people got their first dose. That means the first the first people to get AstraZeneca are scheduled to get their doses by July at the latest.

Because the U.K. is spacing out their doses by 12 weeks there is good data on the fact that protection after one dose of AstraZeneca lasts for at least that long.

How does vaccine supply factor in here?

On the plus side, Ontario has a lot of Pfizer and Moderna doses available, which means they can afford to pause one of the other vaccines while they look into it further.

But the province only has about 50,000 doses of AstraZeneca in storage right now, and according to Dr. Dirk Huyer, co-ordinator of the provincial response, it’s not clear when the next shipment is coming.

India, for example, was tasked with making a lot of the global supply of the Covishield type of AstraZeneca, but has restricted exports due to the outbreak there.

So officials do have some time to research this, but the clock is ticking.

Alex Boyd is a Calgary-based reporter for the Star. Follow her on Twitter: @alex_n_boyd