Merck & Co. (MRK) is facing an increasing number of lawsuits over claims that its shingles vaccine Zostavax causes serious injury and even death. (See also, Merck Blood Cancer Drug Keytruda Wins FDA Nod.)

Shingles, also called herpes zoster, is a condition of reactivation of the chickenpox virus that causes painful rashes that may be accompanied by blisters. Treatments include pain relief and antiviral drugs like aciclovir or valaciclovir. A chickenpox vaccine in childhood or a shingles vaccine as an adult can minimize the risk of developing shingles.

Potential Side Effects: Blindness, Paralysis, Death

Zostavax vaccination has demonstrated effectiveness in reducing the risk of shingles and lowering its severity. However, plaintiffs in state and federal courts in Pennsylvania claim Merck failed to warn them about the severe side effects of the vaccine, reports FiercePharma.

Attorney Marc Bern at Marc J. Bern & Partners claims there are thousands of more cases that are yet to be filed in Philadelphia, as the serious side effects of Zostavax range from blindness in one eye, paralysis, brain damage and death.

In response, Merck defended the safety and efficacy of the vaccine, citing Zostavax's longstanding safety record. The vaccine is approved in over 50 nations, and Merck has distributed over 36 million vaccine shots since Zostavax was approved by the Food And Drug Administration (FDA) in 2006. In March 2011, the indication for Zostavax was expanded to include older adults ages 50 to 59 years.

The drug generated $749 million in revenues for Merck in 2016.

In October 2016, GlaxoSmithKline PLC (GSK) filed for U.S. regulatory approval for its Shingrix which is expected to challenge Zostavax's long-established market share. (See also, Merck Antiviral Drug Reports Positive Data.)