MPX: What We Know

The vaccine being used, Imvamune, is meant to be given as a single dose initially and then as a second dose at least 28 days later. At the moment in Ontario, only one dose is being given so that the supply can be available for everyone who needs their first dose.  

There’s no evidence that waiting longer than 28 days to get the second dose changes how effective the first dose is. Generally, waiting longer between first and second doses of any vaccine leads to a better immune response. And while two is better than one, one is still better than none. So while supplies are limited, the focus is on getting more people their first dose. 

If you have a compromised immune system, then you may be eligible to get a second dose. That’s because your immune response would likely not give you the full benefit of the first dose and you would be more likely to have severe symptoms if you did get MPX.

If you meet the provincial eligibility criteria for getting a second dose (see the Provincial guidance here) and are moderately to severely immunocompromised, then you can go to your local public health unit and get a second dose of the vaccine.  

You’ll need to bring documentation with you, though. If you’re on medication that suppresses your immune system or have a condition that requires medication that’s listed here, then you can bring either the bottle/package that your meds come in or a prescription receipt from your pharmacy. It needs to be from the last six months in order for it to be accepted. 

If you have HIV: If you have HIV and are otherwise healthy, undetectable, and have a high T-cell/CD4 count, then you will have had the full immune reaction needed from the first dose. 

If you have HIV and you have a low T-cell/CD4 count, then you should get a second dose of the vaccine. You will need a note from your doctor that just says that you meet the eligibility criteria as set out by the province to get the second dose. This letter doesn’t need (and shouldn’t include) medical details beyond that. It doesn’t need to say that you have HIV or what your T-cell/CD4 count is. 

For everyone else, the National Advisory Committee on Immunization (NACI) is reviewing their guidance on the vaccine—including second doses for folks with working immune systems—and is expected to have an update in late September. 

In Canada, all Imvamune vaccines are being given in full doses.  

Sometimes, a vaccine can be administered as a partial dose (known as fractional dosing), where only 20% of the volume of a single dose is administered. There’s some evidence that this can be as effective as a full dose, and some evidence that it has a higher chance of causing side effects from the vaccine. 

Authorities in the United States have approved fractional dosing in order to reach more people, given their extremely limited supply of vaccines. In Canada, there has so far been no vaccine shortage for the target population, and so public health officials haven’t approved fractional dosing. 

In Canada, this vaccine is purchased and managed by the federal government through the National Emergency Stockpile. The federal government, informed by the National Advisory Committee on Immunization (NACI), establishes general strategies for how the vaccine will be used in this outbreak response.   

Both the guidelines and the vaccines themselves are then handed off to each provincial or territorial government. These governments are responsible for refining the eligibility criteria for their specific region and for coordinating implementation. The rules for eligibility and administration set out by the provincial government are applied equally across the province.  

These rules and the vaccine doses then get handed off to local Public Health Units (PHUs). The PHU is responsible for setting up immunization clinics, training nurses, and administering the vaccine doses. 

The vaccine being used against monkeypox, called Imvamune, is a specialized vaccine that was developed for people who couldn’t have earlier generations of the smallpox vaccines. Because countries no longer vaccinate the general population against smallpox, there was very low demand for this product and only one manufacturer. 

At the start of this outbreak, Canada was already in a very good position, with doses of Imvamune available in the National Emergency Stockpile. That meant that we were the first country to roll out a large, targeted vaccination campaign to help slow the spread of the virus. 

So far, there has been no issue getting at least a single dose to everyone who is at high risk of being exposed to monkeypox in Ontario, and second doses for those who are immunocompromised.  

We have heard that the federal government has ordered more doses from the manufacturer, but that those will not arrive until 2023.