Here is what officials at the Centers for Disease Control and Prevention and the National Institutes of Health have been telling us: America has some Ebola infections (and is likely to see more), but America does not have an Ebola outbreak, which is extremely unlikely in a health system capable of basic public-health measures (such as isolation and contact tracing).

Here is what officials at the Centers for Disease Control and Prevention and the National Institutes of Health have been telling us: America has some Ebola infections (and is likely to see more), but America does not have an Ebola outbreak, which is extremely unlikely in a health system capable of basic public-health measures (such as isolation and contact tracing).

So: Infections but no outbreak.

Some in the media � splicing their own agendas into the Ebola virus � have used this response (variously) to criticize governmental doublespeak, diagnose institutional rot or slam immigration laxity. This is political and ideological scavenging. It also pits people with limited credibility on this issue against national experts who know their business (as opposed to locals who sometimes don�t).

There is a time to question authority. And then there is a time to shut up and listen carefully to medical experts, who sometimes say complex and difficult things.

Such attentiveness is about to become even more important. The Ebola outbreak in Liberia, Guinea and Sierra Leone (countries lacking effective health systems) is not contained; it is catastrophic. Photographers send back pictures like tourists in hell.

Desperate attention will soon turn to an Ebola vaccine, for a specific reason. The first participant in a Phase 1 trial was injected at the NIH clinical center on Sept. 2. Twenty people are now in the study.

This possible vaccine, as usual, is the result of money, brilliance and luck. Following the 9/11 attacks, the U.S. government began funding research on an Ebola vaccine in earnest (to counter the possibility of bioterrorism). The effort went slowly until about two years ago, when scientists chanced upon a new vector, or method of vaccine delivery: a chimpanzee virus.

Injecting even a dead Ebola virus into a human being (one of the ways to produce an immune response) is too dangerous. So scientists have spliced just one gene of the Ebola virus � the one that codes for its outer coating, called a glycoprotein � into the DNA of a chimp adenovirus. After injection, this harmless virus infects a human cell, which, in turn, pumps out a lot of Ebola glycoprotein. The immune system responds as though the whole Ebola virus � not just the protein for Ebola�s coating � were in the body, producing resistance to future infection. At least theoretically.

By the end of November, we will know two limited things about this proposed vaccine: Is it immediately harmful? (So far, it is not.) And does it produce an immune response?

If the vaccine looks promising after Phase 1, an ethical debate will commence. Some will suggest, as some at the World Health Organization have already suggested, that further testing be bypassed and the vaccine be given immediately to health workers in West Africa. Others may demand that the partially tested vaccine be produced and given out broadly.

This reaction is emotionally understandable. I probably would share it if I lived in a hot zone. But the main response is: the STEP trial.

This was the study of a promising HIV vaccine, which went successfully through Phase?1. But two years into the Phase?2 trial � in which participants were given either the vaccine or a placebo, with no one (researcher or subject) knowing who got what � it became evident that the vaccine resulted in a 41 percent increase in the risk of HIV infection. This �paradoxical effect� (scientists love understatement) also has been seen in other vaccine development efforts.

There is another, apparently pitiless reason that health workers should be involved in a controlled, double-blind Phase 2 study of the Ebola vaccine: because their higher risk of infection would increase the likelihood of the study giving meaningful results (since those results depend on comparing rates of infection within high-risk groups, after some receive the vaccine and others do not).

Why, in times of medical emergency, do we need to trust reliable scientific authorities? Because they are required to do unspeakably difficult things that would not survive the majority vote of a frightened public. In a Phase 2 trial, researchers give some people a vaccine that may hurt them, and some people a placebo that may result in their infection and death. This turns out to be the only ethical way we currently have to determine if a vaccine actually works.

On an Ebola vaccine, this process should be expedited in any way possible. But it can�t be avoided without the potential for great harm.

Michael Gerson writes for the Washington Post Writers Group.