So small that it can be dismissed as a reasonable possibility.

From Flu: The Story of the Great Influenza Pandemic of 1918 and the Search for the Virus That Caused It by Gina Kolata.  Page 135. 

In the meantime, doctors continued to monitor the situation at Fort Dix. Men were still becoming ill with influenza, but almost all of them had the A/Victoria strain. Yet there were troubling signs that swine flu was there. Virologists found swine flu virus in a fifth man who had gotten sick in February and eight men who had recovered from flu had had swine flu, according to blood antibody tests. When doctors at Fort Dix looked for swine flu antibodies among the men, they discovered that as many as 500 of them had swine flu antibodies, indicating that they had been infected by the virus as well.

At the same time, civilians who lived near Fort Dix did not seem to be infected with anything other than A/Victoria. Neither did people living elsewhere in New Jersey. And when the Army looked at other bases they could find no swine flu. Moreover, the National Institutes of Health and state public health officials could not find any swine flu cases among civilians. When the Centers for Disease Control asked the World Health Organization to check for swine flu cases in other countries, the group reported that it could find no evidence of the virus abroad.

Goldfield, speaking about the incident a year later, was frank about the dilemma. “The experience certainly was unusual,” he said. “A radically new strain had appeared in a civilian population and had died out, apparently, the first week of February. It would seem that it hadn’t survived competition with A/Victoria. On the other hand, there has never been a recognition of a radically new strain of A which spread from human to human and did not turn out to be pandemic. The likelihood that the new strain could have started at Fort Dix and was found by us at the first attempt is so small that it can be dismissed as a reasonable possibility.”

Kilbourne said he, too, was conflicted over the failure to find evidence that the swine flu virus had spread. “After Fort Dix, there was a long hiatus when nothing happened; in spite of the fact that there was a hothouse situation in the military barracks, it was not transmitted to the outside community.”