A pedestrian walks outside a Merck & Co. facility in Summit, New Jersey.

A human trial of an experimental Ebola vaccine recently licensed by Merck & Co. has been suspended until January after four healthy volunteers receiving it experienced mild joint pain, according to a Geneva hospital helping conduct the trial.

As a precautionary measure, the study team has declared a pause in the injections, the hospital said. It added that joint pain after infection or vaccination is a well-documented phenomenon which does not worry specialists. However, it deserves to be carefully studied. The hospital said the vaccine overall has been very well tolerated among the 59 people who have received it in Geneva.

Merck agreed in November to pay NewLink Genetics Corp. NLNK, -3.96% of Ames, Iowa, $30 million for the rights to the vaccine, plus an additional $20 million and royalties on sales if the vaccine meets certain development targets.

Merck MRK, -0.32% , of Whitehouse Station, N.J., said in a statement that it was aware the study had been placed on a temporary hold after transient complaints of joint pain in Geneva. These events have not been reported at any of the other clinical sites, the company added. It is not known at this time whether these events are related to the vaccine or not. We understand the level of vaccine being administered in the trial, which is being conducted at a number of other sites, will proceed using lower doses of the vaccine.

NewLink didnt immediately respond to a request for comment.

An expanded version of this report appears at WSJ.com.

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The Wall Street Journal: Human trial of experimental Merck Ebola vaccine is suspended