Genzyme confident Allston plant on line late July

* Says making progress resolving contamination problem

* Product constraints to last 6-8 weeks

* Shares rise 2.9 pct

NEW YORK, June 25 (Reuters) - Genzyme Corp (GENZ.O) said it was confident its Boston plant, shut down after a virus halted production of two of its top-selling drugs, would be back on line next month.

The extensive disinfection process at the Allston Landing plant has begun, the biotechnology company said on Thursday.

The discovery of Vesivirus 2117 forced the maker of treatments for rare genetic disorders to halt production of its Gaucher disease treatment Cerezyme and Fabry disease treatment Fabrazyme earlier this month, leading to shortages of the medicines for which there are few alternatives.

"We continue to make strong progress toward resolving this situation as quickly as possible," Chief Executive Henri Termeer said in a statement. "We are confident that the Allston plant will be back on line later next month, that we will be able to minimize the impact of constrained supply."

Because Cerezyme and Fabrazyme inventories are not sufficient to avoid shortages, the company said it is working with regulatory authorities in the United States and Europe, physicians and patient organizations to manage product supply, with the goal of protecting the most vulnerable patients.

The company currently expects the period of constraint for each product to last six to eight weeks. It said earlier this month that Cerezyme shortages would hit in August and the Fabrazyme shortfall would likely start to be seen in September or October.

Cerezyme was Genzyme's biggest selling product last year and Fabrazyme was No. 3. Together they represented about 37 percent of the company's 2008 revenue of $4.6 billion.

The virus, which was discovered in a bioreactor used for Cerezyme production, has not been shown to cause human infection but is known to interfere with the growth of cells used to produce biologic drugs.

Cambridge, Massachusetts-based Genzyme said the virus was likely introduced through a raw material used in the manufacturing process. It is working with suppliers to address the issue and to implement steps to protect against recurrence.

Genzyme shares were up $1.59, or 2.9 percent, at $56.41 in midday trade on Nasdaq. (Reporting by Bill Berkrot; editing by John Wallace)