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Home » Smallpox vaccine effectiveness for monkeypox — some facts

Smallpox vaccine effectiveness for monkeypox — some facts

I have read several times that the effectiveness of the smallpox vaccine against monkeypox was over 85%. The number comes from the CDC website information about monkeypox which most of us take as authoritative.

I know that I’m probably rushing a little bit to talk about monkeypox and a potential vaccine, given that there have been only two hundred confirmed and suspected cases in the world (as of the date of this article), but there are some troubling issues with this outbreak including a much higher infectivity rate.

If those of us who have received the smallpox vaccine (which was at least 40 years ago for the youngest of us) retain 85% effectiveness against monkeypox, then I’m going to worry a lot less. However, even though that number was posted by the CDC, they gave no links to peer-reviewed articles that support that number. And it was unclear whether they meant the modern smallpox vaccines or the ancient vaccine that nearly 100% of us received decades ago.

So, I’m going to dig into it because I think we should know. Plus the more accurate information we have, the better we are going to deal with the inevitable anti-vaccine tropes, memes, and outright lies that will soon appear across the internet.

closeup photo of primate
Photo by Andre Mouton on

What is monkeypox?

Monkeypox is caused by the monkeypox virus which is a double-stranded DNA, zoonotic virus. It is one of the human orthopoxviruses that includes variola (smallpox), cowpox, and vaccinia viruses. But it is not a direct ancestor to, nor a direct descendant of, the variola virus which causes smallpox. Monkeypox virus causes a disease that is similar to smallpox but with a milder rash and lower death rate.

The monkeypox virus is endemic to the tropical rainforests of West Africa and is found in primates and some other animals, such as the Gambian pouched rat.

According to the CDC, symptoms begin with fever, headache, muscle pains, swollen lymph nodes, and feeling tired. This is followed by a rash that forms blisters and crusts over. The time from exposure to onset of symptoms is around 10 days. The duration of symptoms is typically two to four weeks. The symptoms are similar to chickenpox, although the Varicella zoster virus that causes chickenpox is not related to the monkeypox virus.

Transmission of monkeypox virus occurs when a person comes into contact with the virus from an animal, human, or materials contaminated with the virus. The virus enters the body through broken skin (even if not visible), respiratory tract, or mucous membranes (eyes, nose, or mouth). Human-to-human transmission is thought to occur primarily through large respiratory droplets, but there is some concern that this new outbreak of monkeypox can be transmitted through aerosol droplets which can make it more infectious.

The mortality rate for the disease ranges from 1% to 10%, depending on the variant (yes, we’ll be talking about variants again).

Monkeypox can be treated with anti-viral medications such as cidofovir. There may be other anti-viral drugs that can be used, but they have not been tested against this disease as of this time. You can bet there will be a lot of published articles on new drugs for monkeypox in the next few months.

The monkeypox vaccine

There is no vaccine for monkeypox itself, but there is a vaccine (for smallpox) that has been shown to be effective against the monkeypox virus. The vaccine, JYNNEOS (known as Imvamune in Canada and Imvanex in the EU) is an attenuated live virus vaccine, manufactured by Bavarian Nordic, that has been approved by the U.S. FDA for the prevention of monkeypox.

The Advisory Committee on Immunization Practices (ACIP) is currently evaluating JYNNEOS for the protection of people at risk of occupational exposure to orthopoxviruses such as smallpox and monkeypox in a pre-event setting. I’m sure this discussion will move to near the top of the list in future ACIP meetings. The vaccine is also approved in Canada and the EU for smallpox and monkeypox.

The USA has ordered 13 million doses of the vaccine for delivery in 2023 and 2024 probably for use for military personnel and those individuals who are researching these viruses. However, I am certain that if this outbreak grows to a few thousand individuals, all countries will be scrambling to order more of the vaccine and speed up deliveries.

monkeypox vaccine

Effectiveness against monkeypox

As I wrote above, the CDC is reporting that the smallpox vaccine is 85% effective against the monkeypox virus. However, I wanted to find out what evidence supports that claim, which vaccines were included in that claim, and whether the original smallpox vaccine, last given widely in the mid-1970s, has been shown to have any effectiveness against the new virus.

Let’s look at some of the evidence:

  • A 2005 article showed that the currently available smallpox vaccine-induced antibodies were effective in protecting against the monkeypox virus. However, this research was in rhesus monkeys and did not establish whether the antibodies remain from those of us who were vaccinated 40-50 years ago.
  • A 2004 study showed that the MVA smallpox vaccine was very effective against the monkeypox virus. But again, this did not provide any data as to the effectiveness against monkeypox for individuals vaccinated in the 1970s.
  • A small study of 246 participants showed that 97% had no decrease in vaccinia-specific antibody titers over a follow-up period of up to 88 years. 

I could list several other articles that show that the older and newer smallpox vaccines are effective against monkeypox. The fact that the FDA (and EMA) have approved these vaccines for protection against monkeypox means that preclinical and clinical trials show that they are effective.

But I cannot find any definitive information that shows that those vaccinated in the 1960s and 70s (and earlier) are immune to monkeypox. Fortunately, that one small study did show that antibody levels against smallpox remained after decades, so we might be able to assume that those of us who were vaccinated against smallpox are also protected against monkeypox.

However, only about 30% of the world’s population has ever received a smallpox vaccine, so even if the 85% could be applied to those of us that did receive the vaccine decades ago, monkeypox would still be a danger to 70% of the people out there.

In fact, the successful eradication of smallpox can “…invite speculation that emergent or re-emergent human monkeypox might fill the epidemiological niche vacated by smallpox.” No, this does not mean it was a bad idea to eradicate smallpox, which has a 30% mortality rate, it just means that it made room for another orthopoxvirus to fill the ecological niche.

Anyway, as far as I can tell, the “85% effectiveness” number touted by the CDC is for those who receive the newer smallpox vaccines. It does not give any indication of the effectiveness of the vaccine given in the 1970s smallpox vaccine cohort. However, I think that preliminary evidence shows that those of us who got that smallpox vaccine may be protected against monkeypox, though, again, I do not have definitive evidence supporting that opinion.

My guess would be, in the abundance of caution, that if this outbreak grows to a pandemic (please, NO), everyone would need the new smallpox vaccine. However, based on no evidence whatsoever, I would make a scientific guess that those of us who did get the smallpox vaccine so long ago would have some level of protection against the monkeypox virus.


There is so much we don’t know right now. Are we overreacting to monkeypox because our radar is so attuned to any infectious disease outbreak these days? How infectious is monkeypox (it seems to be more infectious than earlier outbreaks)? What is the mortality rate?

As for a monkeypox vaccine, we have that in the form of the smallpox vaccine. That’s the good news. Unfortunately, there is not a large supply of it across the world. That’s the not-so-good news.

As for that “85% effectiveness” — that only applies to de novo vaccinations, not for those who were vaccinated 40 or 50 years ago, although the smallpox vaccine from decades ago can still protect against smallpox and, presumably, monkeypox. However, even if it is still effective, that only applies to a small minority, around 30%, of the world’s population.

I hope this story goes away. But humanity hasn’t had that sort of luck for the past two years.


Michael Simpson

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