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Home » Monkeypox vaccine effectiveness — we don’t know much

Monkeypox vaccine effectiveness — we don’t know much

There’s a lot of information on various news sites about the effectiveness of the monkeypox vaccine. They often claim it’s 80% or higher. Unfortunately, I accepted those numbers as a “scientific fact,” so I decided to dig into what supported the claims of the effectiveness of the monkeypox vaccine. I was surprised by what I found.

I think when something like monkeypox, or the novel coronavirus two years ago, we jump on early data without analyzing it properly. And that’s where we stand with the monkeypox vaccine — exactly how safe and effective is it?

I’m not going into the safety of the vaccine at thiscause I don’t have good data to give you. In fact, I don’t have really good data to give you about the vaccine’s effectiveness, and that’s my point.

So, let’s take a look at the evidence that has been published on the effectiveness of the monkeypox vaccine.

Monkeypox virus

About monkeypox and the vaccine

Just as a refresher, 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. Despite the name, monkeypox isn’t really a disease specifically associated with monkeys.

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 much of the current outbreak appears to be from sexual contact, and may eventually be considered a sexually transmitted disease (STD).

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.

As of July 29, the CDC reports that the current outbreak has caused 5,189 infections in the U.S., striking nearly every state (all except Montana, Wyoming, and Vermont) plus Puerto Rico. New York is by far the most affected with 1,345 cases, followed by California with 799, Illinois with 419, Florida with 373, and Texas and Georgia with 351 apiece. There’s an especially high per-capita rate in Washington D.C., with 218 cases in a city of 702,000 people.

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 the U.S. FDA has approved for the prevention of monkeypox.

Monkeypox vaccine effectiveness

The number that’s been frequently quoted by media organizations is “85% effective” or “at least 85% effective.” But when I dig into that number, it is apparently based on one small study done in Africa in the 1980s that has major limitations. Most of the other studies have only been conducted on animals, and unless you don’t read any of my articles, you would know that I take animal studies with a grain of salt.

I even got fooled by this 85% effectiveness when I wrote about the vaccine just a few weeks ago.

According to the CDC, there is “no data are available yet on the effectiveness of these vaccines in the current outbreak.” The CDC is generally careful about how they describe new vaccines, but that’s a pretty clear description of the vaccine.

As I discussed previously, the claims of 85% effectiveness of the monkeypox vaccine are based on a small study, a retrospective analysis published in September 1988 in the International Journal of Epidemiology.

In that study, researchers tracked the household contacts of 209 people infected with monkeypox in Zaire in the early 1980s. Those with scars from previous smallpox vaccination (70%) were 85% less likely to be infected. The vaccine seemed to be 89% effective at protecting contacts outside the household from infection.

Here are my concerns about the study:

  • It was tiny. Remember, most clinical trials for vaccine safety and effectiveness include thousands of patients. The clinical trials for the COVID-19 vaccines had around 40,000 subjects in each.
  • It was a retrospective cohort study, not a randomized, double-blind clinical trial.
  • It was demographically narrow, containing only subjects in Zaire.
  • The statistical analysis was rather odd, not containing any confidence intervals.
  • It had no adjustment for other factors such as age and sex.
  • It only looked for signs of previous vaccination, the smallpox vaccine scar, and it did not include any information about the immune status of the subjects.

This one study is it. There are no other data that show whether the vaccine has any level of effectiveness against the virus. And this study was done based on the older smallpox vaccine, which many of us got in the 1960s and 70s, and not with the new JYNNEOS vaccine. We know even less about its effectiveness against the monkeypox virus.

In fact, JYNNEOS has not been tested against smallpox or monkeypox in clinical trials. Now there are a couple of reasons for this — smallpox is essentially eradicated from the planet and monkeypox wasn’t endemic until recently. There is a large body of animal studies that support the effectiveness of the vaccine, but not in humans. Many scientists roll their eyes at animal studies that have not been confirmed by clinical trials. I’m one of those.

But there’s one more issue that could be problematic. These vaccines were developed based on the assumption of skin-to-skin contact like we used to see with smallpox a generation or two ago.

The current monkeypox outbreak appears to be mostly sexually transmitted— the head of the penis and interior of the anus may have characteristics that make them more susceptible to infection such as a lower dose of virus needed, less abundant antibodies or other components of the immune system, or concurrent STDs increasing risk.

This may not be a concern, but we won’t know until there is adequate testing.


At this time, what we know about the effectiveness of the JYNNEOS vaccine against monkeypox is really limited. And a lot of public health mitigation seems to be focused on the vaccine rather than the prevention of the disease, although, to be fair, many are trying to do both at the same time.

I am troubled by the claim that the vaccine is 85% effective against monkeypox. The evidence used is really weak, and it would never be used as the basis of an FDA application for approval.

That does not mean the vaccine is ineffective, it’s just that we don’t know how effective it is. If it really has poor effectiveness, then we need to be working on an improved vaccine at the speed at which we got new COVID-19 vaccines. Without this data, public health officials are guessing at how best to attack this monkeypox outbreak.

Would I get the vaccine if I were a gay male? You bet I would, but because I know the effectiveness is unknown, I’d still follow public health advice and avoid situations where I might get infected.

This is why I get frustrated with medical journalism today — they make assumptions that are not supported by science. Right now, I cannot tell you what the effectiveness of the JYNNEOS vaccine is — it could be 5% or it could be 95%. No data supports any claim of effectiveness.


Michael Simpson

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