(Adds shares in paragraph 2, details on disease in paragraph 5 and 9, details on trial findings in paragraphs 6-8)

Dec 10 (Reuters) - Siga Technologies' antiviral drug did not reduce the time to lesion resolution or have an effect on pain among adults affected by the clade II strain of mpox, according to a U.S. National Institutes of Health study published on Tuesday.

A data safety and monitoring board recommended stopping further enrollment of patients in the study based on the interim results. Shares of the company closed 6.7% lower at $6.47.

The trial, sponsored by NIH's National Institute of Allergy and Infectious Diseases (NIAID), began in September 2022 as part of the government's response to the clade II mpox outbreak.

It enrolled patients who had been ill with mpox for less than 14 days in Argentina, Brazil, Japan, Mexico, Peru, Thailand and the United States, including Puerto Rico.

Mpox is caused by a virus that spreads mainly through close contact. A clade II subtype virus caused a global mpox outbreak in 2022 and continues to circulate at low levels.

A planned interim analysis at 75% of the study's target enrollment showed there was no difference in the time to lesion resolution among participants treated with Siga's drug tecovirimat, branded as TPOXX, compared with those who received a placebo.

The analysis also showed pain decreased similarly among patients treated with tecovirimat and those who received a placebo.

This year, an outbreak of the clade I strain of mpox was declared a public health emergency of international concern by the World Health Organization.

(Reporting by Sriparna Roy in Bengaluru; Editing by Pooja Desai and Krishna Chandra Eluri)

((Sriparna.Roy@thomsonreuters.com;))