As the number of confirmed monkeypox cases continues to rise in Canada, infectious disease and public health experts are providing vaccinations to those at risk of infection.

Since monkeypox and smallpox are both part of the orthopox family of viruses, experts say some vaccines will work against both.

But the vaccines being recommended for use against monkeypox today are different from those used in the last century's global effort to eradicate smallpox, the World Health Organization (WHO) says.

"Some countries have maintained strategic supplies of older smallpox vaccines from the Smallpox Eradication Programme (SEP) which concluded in 1980," said the WHO's interim monkeypox vaccination guidance issued on Tuesday.

"These first-generation vaccines held in national reserves are not recommended for monkeypox at this time, as they do not meet current safety and manufacturing standards."

Monkeypox causes flu-like symptoms and skin lesions, and spreads through close contact.

WATCH | WHO asks countries to collect data on smallpox vaccines used against monkeypox:

Most of the data on using smallpox vaccinations against monkeypox is old, or comes from animal studies rather than human clinical trials, said Dr. Rosamund Lewis, the WHO's technical lead for monkeypox, at a briefing on Tuesday. 

Lewis called for countries immunizing people against monkeypox to track and share their findings so they can learn more about how effective the vaccines are. 

Both WHO and Canada's National Advisory Committee on Immunization (NACI) have now released guidelines on what vaccine to use against monkeypox, and who might benefit from it.

The vaccine approved for immunization against monkeypox in Canada is MVA-BN, or Modified Vaccinia Ankara - Bavarian Nordic. Bavarian Nordic, headquartered in Denmark, is the company that manufactures it.

In Canada, the vaccine has the trade name Imvamune. (It's called Imvanex in the European Union and Jynneos in the U.S.)

Imvamune was originally authorized in Canada for "extraordinary use" against smallpox in November 2013, as part of the federal government's emergency plan to immunize people if the deadly disease were ever to resurface. In 2020, Canada expanded the vaccine's authorization to include immunization against monkeypox, NACI documents say.

Routine smallpox vaccinations stopped in Canada in the early 1970s. But because the viruses are related, those smallpox vaccinations may have provided some degree of immunity against monkeypox as well, experts say.

"It's likely that this, you know, mass immunization campaign for smallpox really kept monkeypox in check for many years," said Dr. Isaac Bogoch, an infectious diseases specialist at Toronto General Hospital.

But most people under 50 in Canada didn't get that protection — and that might be one of the reasons we're seeing monkeypox now, he said.

Imvamune contains a weakened strain of the vaccinia virus, which provokes the immune response to fight off smallpox and monkeypox.

Unlike the original smallpox vaccine, this weakened virus is non-replicating, meaning it can't make copies of itself.

Early-generation vaccines based on replicating vaccinia strains led to unpleasant side effects for some and could be problematic for people who were immunocompromised, said Dr. Zain Chagla, an infectious disease specialist at McMaster University in Hamilton.

In addition, older generations of smallpox vaccine are administered differently. Instead of a regular syringe, the health-care provider uses a "bifurcated needle" with two prongs and pricks the skin repeatedly to get the dose in.

Infectious disease specialists say, right now, monkeypox is not affecting the majority of the general Canadian population, although anyone can get the virus if they're in close physical contact with someone who is infected.

The LGBTQ community, particularly men who have sex with men, are at disproportionate risk of infection in this outbreak, they say. Health-care workers or lab workers who work directly with orthopoxviruses could also be at risk.

As a result, experts say monkeypox vaccinations should be offered to those at-risk populations as a preventative measure. The vaccine can also be offered to those who have already been exposed to someone with monkeypox — known as post-exposure prophylaxis.

In both cases, monkeypox immunization outreach needs to be targeted, they say.

"This is not a widespread vaccine for everyone; it is risk-based vaccination," Chagla said.

"I don't think we're anywhere near starting, you know, vaccine campaigns for the general public," he said.

"At this point in time, this should be a very focused and targeted vaccine campaign for people who have been exposed and for people who are at the highest risk of getting this infection."

Both the WHO and NACI recommend two doses of Imvamune, given 28 days apart.

A possible exception, according to NACI's guidance, is someone who has had a smallpox vaccination in the past. In that case, they could be given just one dose to act as a booster.

Yes. NACI says Imvamune includes the following ingredients:

Traces of residual host (egg) cell DNA and protein.

Nicole Ireland is a CBC News journalist with a special interest in health and social justice stories. Based in Toronto, she has lived and worked in Thunder Bay, Ont.; Iqaluit, Nunavut; and Beirut, Lebanon.

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