Why vaccines are so slow

Early news about medical treatments — like yesterday’s announcement that a coronavirus vaccine has shown positive results in eight people — can feel both exciting and frustrating.

The frustrating part is the timing. Even if all continues to go well with the research into this vaccine, it won’t be available until late this year or early next year. Between now and then, the vaccine will have to endure two more research trials, one involving hundreds of people and the other involving thousands.

Given the virus’s terrible toll, that long process can seem strangely lacking in urgency. But scientists insist that it isn’t. Here are the key reasons they say that there are no easy or fast routes to a vaccine:

Early results don’t always stand. In 2015, the French drug company Sanofi began selling the first vaccine for dengue. The drug had made it through multiple research trials — although some researchers believed Sanofi had ignored worrisome signs. Sure enough, as children in the Philippines began using it, some contracted an even worse form of dengue. Today, use of the vaccine is highly restricted.

In recent testimony, Dr. Anthony Fauci, the nation’s top infectious-disease expert, noted that a coronavirus vaccine could suffer from the same problem.

The larger point is that drugs that look good in small, initial studies often look less good when they’re tested in more people.

Side effects matter. A vaccine doesn’t merely need to work, as Katie Thomas, a Times reporter covering pharmaceuticals, explained to me. It needs not to have side effects that cause more damage than the virus itself.

This coronavirus seems to kill only a small percentage of people who get it. The side effects have the potential to do more damage, because any coronavirus vaccine will be given to billions of people, including many with underlying health problems.

Politics matter, too. Vaccines are the subject of frequent conspiracy theories and falsehoods. Given this skepticism, a coronavirus vaccine that did more harm than good could cause much broader damage.

It could lead people around the world to stop taking vaccines that actually work. That’s what happened in the Philippines after the dengue scandal.

All of which is a reminder that promising early results — like yesterday’s — often prove fleeting. Only about 10 percent of drugs that clear the first research phase ultimately make it to market.

The New York Times Morning Briefing