US Merck’s shingles (herpes zoster) vaccine Zostavax continues to be the target of legal action at a time when the drugmaker is already facing the prospect of competition from GlaxoSmithKline’s (GSK) vaccine Shingrix.

In the latest round, 18 patients from the US states of Florida, Kentucky, Louisiana, Michigan, Mississippi, Missouri, New Hampshire, South Carolina, Tennessee, West Virginia, and Wisconsin filed suit against Merck on Jul. 11. They allege the company produced and sold an “unreasonably dangerous vaccine” that caused them to develop a “persistent strain” of the disease it was designed to treat.

California-based pharmaceutical distributor McKesson and a member of Merck’s vaccination team are also named in the suit.

Altogether, the claim filed in Middlesex County Superior Court, near the drugmaker’s headquarters in Kenilworth, New Jersey, points to linked to 36 deaths and 1,111 serious adverse events surrounding Zostavax it says were reported to the FDA up to September 2015. The vaccine, recommended for patients above age 50, is licensed in 50 countries.

Attorneys for the patients inoculated with the live virus accuse Merck of negligence, defective design, failure to warn, breach of express and implied warranties, along with misrepresentation involving risk of physical harm and unjust enrichment. They are calling for a jury trial and a judgment to include punitive damages, claiming that the company knew, or should have known, that its product caused viral infection and did not adequately warn of the risk.

Zostavax was approved by the US Food and Drug Administration in 2006, but herpes zoster was not added to the list of side effects until August 2014

Earlier this year, a separate group of patients and their families, most of them in the state of Pennsylvania, filed sued against Merck for a range of alleged complications from the vaccine. This litigation is still pending.

Zostavax had sales of $749 million in 2015 but it will soon face a challenge from GSK. Analysts have forecast that Shingrix, expected to be approved by the FDA later this year, could have annual sales as high as $1 billion by 2022.