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With growing concerns over Zika virus, you may have read about a vaccine on the way. A word of advice: Go find a big grain of salt.

It’s true that multiple major pharmaceutical companies and many minor ones have announced plans to either make a Zika vaccine or assess the prospects of doing so.


Companies that have designed vaccines for similar viruses are studying the possibility of adapting their blueprints to make a Zika vaccine.

That, though, is like figuring out if a recipe for oatmeal cookies can serve as the basis for Toll House cookies. Building on previous work should cut down on the time it takes to make a Zika virus vaccine. But it’s not as simple as switching out raisins and adding chocolate chips.

Here’s some context to consider when you’re reading about companies’ claims on their progress toward a Zika vaccine:


Candidate vaccine vs. commercial vaccine

For most of us, a vaccine is a disease-preventing product that has been approved by regulators and made available through doctors or pharmacists.

But to the people who write press releases for biotech companies, “a vaccine” can mean something else. Case in point: News articles that reported this week that an Indian firm called Bharat Biotech International had developed the world’s first Zika vaccine.

Big catch: It hasn’t yet been tested in animals or people.

“A lot of times people will say ‘Well, we have a vaccine’ based on maybe having what we … should better think of as an early vaccine candidate,” said Dr. Jesse Goodman, a professor of infectious diseases at Georgetown University in Washington and a former chief scientist at the Food and Drug Administration.

Are you at risk for contracting Zika virus? Your level of risk depends in part on your living conditions. Alex Hogan/STAT

Remember that term: candidate vaccine. Anything anyone is talking about now is not a vaccine in the consumer sense of the word. It is an experimental vaccine — that is, a vaccine candidate.

There’s a very long and treacherous path from this early stage of development to a product that the FDA would allow doctors to inject into the arms of patients. The perils of this road are so famous, in fact, that it has a name.

Let’s talk about the Valley of Death

That’s a term used to describe what happens between “Eureka!” and “Roll up your sleeve, please.” It is carcass-strewn; many start, but few finish the journey.

“It’s safe to say the majority of things don’t end up being approved products,” said Goodman, who just joined the board of one of the world’s largest vaccine manufacturers, GlaxoSmithKline.

There are a variety of reasons why so many candidates get discarded.

Sometimes, a vaccine candidate simply doesn’t work well enough. Or early testing — generally in animals — raises safety concerns. Other times, there may not be enough of a need; candidate West Nile vaccines have been developed, but pharmaceutical companies haven’t pushed forward with them because they don’t believe there is sufficient market demand.

In other cases, the demand may be there, but the people who need the vaccine can’t afford to pay much. That’s why there is still no licensed Ebola vaccine, though prospects look much better in the wake of West Africa’s heartbreaking two-year-plus outbreak.

Even Valley of Death survivors have a long road to travel

In basic terms, a candidate vaccine has to go through toxicity testing to make sure it’s safe to use. That is first done in animals. Other animal studies look at whether the experimental vaccine works.

(A looming problem for Zika vaccines is that currently, it’s not known what scientists should look for when trying to gauge if they are protective. It’s not as simple as seeing more antibodies in a vaccinated person’s blood; researchers need to know which parts of the immune system the vaccine needs to activate to be effective against Zika. Right now they don’t.)

After animal testing, comes Phase 1 trials in people. These are very small studies involving tens of hundreds of people, Goodman said. They don’t prove a vaccine works. They help manufacturers determine the proper dose for a vaccine and whether it’s safe to continue testing it in people.

Phase 2 and 3 trials involve many more volunteers in an effort to discern if a vaccine is protective. If those trials are successful, and the market conditions look favorable, a manufacturer could apply to a regulator for a license to sell the vaccine.

Goodman said the vaccine world is getting better at shaving time off this process for the development of vaccines for new disease threats that trigger emergency responses. But it still takes time.

What’s his guess for how quickly a Zika vaccine could become commercially available?

“Oh, boy,” Goodman said with a sigh. “Let’s put it this way: My ambition would be that for important emerging infectious diseases that we can go from discovering a [disease] agent to getting an approved vaccine in a matter of a few years rather than 10 to 20 years.”