A Quick Primer On The New Ebola Vaccine

The news headlines have proclaimed that the new Ebola vaccine is “100% effective”. That’s not an accurate statement. This post will summarize the most important information about the vaccine, and correct some inaccuracies in news sources.

Here is the medical study, published in the journal The Lancet: Efficacy and effectiveness of an rVSV-vectored vaccine in preventing Ebola virus disease: final results from the Guinea ring vaccination, open-label, cluster-randomised trial

Let’s start with the effectiveness of the vaccine. The Lancet article says: “Vaccine efficacy was 100% (95% CI 68•9–100•0, p=0•0045)” for the randomized portion of the trial. Persons at risk for Ebola were randomly assigned to receive the vaccine, either immediately, or after a delay. Once the vaccine proved to be effective, they vaccinated everyone. And for that second portion of the trials: “The estimated vaccine efficacy here was 100% (95% CI 79•3–100•0, p=0•0033).”

So the article says “100%” effective, because no one who receive the vaccine later became infected with Ebola. However, the “95% CI” is the Confidence Interval based on a statistical analysis of the data. There is a 95% chance that the vaccine is 68.9 to 100 percent effective, according to the first trial, and 79.3 to 100 percent effective for the second trial. In other words, if the vaccine were given to a much greater number of persons, there is a good chance that the effectiveness would fall somewhere between 68.9 and 100 percent. The researchers are not really claiming an unqualified 100% effective vaccine. No vaccine is 100% effective across a large and diverse population.

A second important point is that the vaccine only has the aforementioned degree of effectiveness for the Zaire Ebolavirus strain. The vaccine might be less effective against other Ebola strains. And it is entirely possible for a strain of Ebola to develop in the wild that is immune to the vaccine.

How does the vaccine work?

Some vaccines contain the live attenuated virus. The famed Sabin polio vaccine was a live attenuated vaccine. In such a case, you are actually being injected with a weakened version of the viral disease you are hoping to avoid. With a live vaccine, there is some risk of infection.

The subsequent Salk polio vaccine was an inactivated vaccine, meaning that the virus is killed and then used for injections. The Salk vaccine was more effective because it included three different version of the polio virus, so that immunity would develop to all three. You can’t give someone three attenuated versions of the same disease all at once; the risk of infection would be too great.

Well, no one wants to be injected with either an attenuated or killed Ebola virus. The disease is too severe. So the new Ebola vaccine uses a more recent vaccine technology: recombinant DNA. The DNA code for one protein found on the surface of the Zaire Ebola virus is spliced into a virus that afflicts animals, but not people: the Vesticular Stomach Virus (VSV). Then that live (and not attenuated!) virus is used as the vaccine. Your immune system reacts to the Ebola protein produced by the harmless virus, and you therefore develop immunity to Ebola.

The vaccine is owned by the Public Health Agency of Canada, and is now licensed to Merck, a large pharmaceutical company. The vaccine is grown in cells from African green monkeys. If a large amount of the vaccine were suddenly needed, Merck has the resources to make a fair amount of the vaccine. However, the necessity of using monkey cells would limit and slow production, regardless of the financial and technical resources offered by Merck.

“Past epidemics have exposed supply chain problems. But the World Health Organization says Merck has offered assurances that it can ramp up production if needed, and the global vaccine alliance known as GAVI has earmarked $300 million for the purchase of several million doses.” [Washington Times]

Those several million doses do not exist, as of yet. I certainly hope that millions of doses are produced and stockpiled. Ebola is one of the worst diseases to afflict humanity. But I am not reassured by Merck’s claim to be able to ramp up production. It is a difficult vaccine to make.

What if Ebola affects the United States? Of course, we must have a stockpile of the vaccine for Africa, where the recent outbreaks have occurred. But it is a small world. An outbreak in the U.S. is not out of the questions. To make an outbreak in the U.S. less likely, we should vaccinate lab workers who might exposed to Ebola, as well as any health care workers who travel to an area with an Ebola outbreak.

The aforementioned vaccine trials used a “ring” approach to vaccination. If one person is at risk, due to contact with an infected person, they vaccinated that person and all the “contacts” in their life (family, friends, neighbors, co-workers). In some cases, they vaccinated contacts of contacts. You literally had to know someone to get the vaccine. The reason was to give the vaccine to persons who were most likely to be exposed to the virus.

What if there were a major outbreak of Ebola in the U.S.? There are 325 million persons in America. You can’t stockpile that much vaccine. The cost and logistics would be almost insurmountable. They would certainly vaccinate health care providers. (Maybe it’s time to get an EMT certification.) And they might use the ring approach and vaccinate their “contacts”. But there would not be enough vaccine for everyone and anyone to get it.

Imagine being told, during such an outbreak, that you can’t get the vaccine. It’s like not being on the list for a private club. I suppose some doses of the vaccine would be stolen; some might be available on a black market. Desperate people do desperate things. Let’s hope is never comes to that.

– Thoreau

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