The risk posed by monkeypox is low, but nearly everyone in Canada is susceptible because routine vaccination against smallpox ended decades ago, top public health officials said Friday.
Monkeypox is a rare disease that comes from the same family of viruses as variola. That virus causes smallpox, which the World Health Organization declared eradicated around the globe in 1980. It is also linked to the vaccinia virus used in the smallpox vaccine.
The Public Health Agency of Canada, which is investigating about two dozen possible cases of monkeypox on top of two confirmed cases in Quebec, says it is spread through prolonged close contact. That includes through direct contact with an infected person's respiratory droplets, bodily fluids or sores, and is not very contagious in a typical social setting.
The B.C. Centre for Disease Control said Friday that it is not investigating any suspected cases or possible contacts of monkeypox in the province after having ruled out two potential contacts.
Canada's chief public health officer Dr. Theresa Tam said the federal public health agency does not know how widespread the disease has become in the country.
Monkeypox is typically milder than smallpox and can cause fever, headache, muscle aches, exhaustion, swollen lymph nodes and lesions all over the body.
There is global evidence that smallpox vaccines can offer protection against monkeypox.
But Canada stopped routinely immunizing people against smallpox in 1972.
Tam's deputy, Dr. Howard Njoo, said this means everyone is susceptible to monkeypox.
“I would say, generally, the entire population is susceptible to monkeypox,” Njoo said Friday.
Canada does keep a small stockpile of smallpox vaccine in case of a biological incident, like a laboratory exposure.
A smattering of cases in the United Kingdom prompted that country to begin offering the vaccine to health workers and close contacts of confirmed cases.
Tam said Canada is considering a similar strategy.
“Quebec had some interest in terms of the contacts so that is under discussion right now, but of course we need to know some of the epidemiology as quickly as possible,” Tam said.
She would not say how many doses of the smallpox vaccine Canada has available, citing security reasons.
Public Services and Procurement Canada put out a tender last month to purchase 500,000 doses of the Imvamune smallpox vaccine on behalf of the Public Health Agency of Canada from 2023 to 2028.
"Although smallpox disease is currently considered to be eradicated, PHAC is procuring a stockpile of the vaccine to immunize Canadians against smallpox disease should a risk ever arise where smallpox is intentionally or unintentionally released," the tender read.
The product monograph that Health Canada posts on its website for Imvamune, from Danish biotech company Bavarian Nordic, says it can be used for both smallpox and monkeypox.
The company announced Thursday it had secured a deal with an unnamed European country to supply its vaccine in response to monkeypox cases.
There is still a sense of mystery surrounding the sudden appearance of the virus in Canada, the U.S., Australia and several parts of Europe.
"Not many of these individuals are connected to travel to Africa where the disease is normally seen, so this is unusual. It's unusual for the world to see this many cases reported in different countries outside of Africa," Tam said.
Canadian health systems are casting a wide net in their search for more cases, she said, because there is not enough known about why the virus is suddenly cropping up around the globe.
"There's probably been some hidden chains of transmission that could have occurred for quite a number of weeks, given the sort of global situation that we're seeing right now," she said.
Njoo said global public health authorities need to be open to the idea that monkeypox is evolving, and transmission may have changed as well.
For now, samples from suspected cases are being sent to the National Microbiology Laboratory in Winnipeg, but PHAC is working with provinces to set up more local diagnostics.