Health Law Daily Wrap Up, PRODUCTS LIABILITY (DRUGS)—E.D. Pa.: Merck succeeds in excluding expert’s opinion on specific causation in bellwether case involving shingles vaccine, (Oct 13, 2023)

By Wendy Biddle, J.D.

When the doctor failed to rule out idiopathic causes of GBS, he did not have good grounds for his specific causation opinion.

In a bellwether case concerning the side effects and efficacy of Zostavax, a vaccine for preventing shingles, the federal district court in Philadelphia denied the drug maker’s motion to exclude two expert witnesses’ general causation opinions, finding that the experts met the standards under Federal Rule of evidence (FRE) Rule 702 that Zostavax can cause Guillain-Barré syndrome (GBS). However, the court did grant Merck & Co.’s motion to exclude one of the expert’s specific causation opinions because the opinion did not meet the reliability standards of Rule 702 and Daubert (Bokus v. Merck & Co. Inc., et al., October 11, 2023, Bartle, H.).

Background. The plaintiff alleged that he developed Guillain-Barré syndrome (GBS) after receiving the Zostavax vaccine to prevent shingles. The patient was 50 years old when he received the vaccine. Three days after being inoculated, he began experiencing fatigue and tingling in his arms and feet. Later he experienced progressive weakness, hand numbness, back pain, and delayed reflexes.

He was admitted to the hospital and was suspected to have GBS. He received intravenous immunoglobulin treatment and responded well. Although he still experiences weakness in his legs and feet, his strength and function improved along with his reflexes.

GBS is a relatively rare autoimmune disorder that causes one’s immune system to attack its own nerves. The cause of the disorder is not well understood and it affects between one or two people per year for every 100,000 people. One-third of all cases of GBS have an unknown cause.

Expert witnesses. The plaintiff had two expert witnesses that gave their opinion on causation, both general and specific. Dr. Poznansky is a Harvard Medical School professor and has practiced infectious disease medicine and immunology for nearly four decades. Dr. Saperstein is a board-certified neurology and neuromuscular medicine physician and a Professor of Neurology at the University of Arizona Medical Center. Dr. Saperstein has twenty-five years of experience treating immune-related neuropathies, such as GBS.

Merck filed a motion to exclude both witnesses, not on their qualifications, but rather the reliability of the methods the experts used in reaching their causation opinions.

General causation opinions. Both doctors opined that Zostavax causes GBS through a process called molecular mimicry. The theory of molecular mimicry hypothesizes that a live-attenuated virus can cause the immune system to create not only antibodies that respond to that virus but also proteins that attack one’s own immune system.

Both doctors relied on medical literature that supports the existence of molecular mimicry as well as studies that concluded that molecular mimicry can cause GBS. Merck argued that the theory of molecular mimicry demonstrates only the biological plausibility and is insufficient to prove general causation.

Although there are no epidemiological studies that support the theory that molecular mimicry occurs in the Zostavax vaccine, the doctors both relied on a study comparing the incidence of GBS following inoculation with Zostavax to that of Shingrix, a competing shingles vaccine. This study, known as the Goud study, used Zostavax as a baseline to determine whether Shingrix carries a greater risk of causing GBS than Zostavax does. The study also reported the incidence of GBS in the general population: one to two cases in the United States per year per 100,000 persons. The study concluded that GBS occurred at a rate of 4.36 cases per 100,000 persons per year who received the Zostavax vaccine. Dr. Poznansky stated the increased risk of GBS from Zostravax is calculable and GBS occurs two to four times more frequently following Zostavax than in the general population.

Merck argued that the doctors failed to account for the Lai study, which stated that the odds of contracting GBS from Zostavax were not significantly increased post immunization. However, the reliability of the Lai study depends on its statement that “Tetanus toxoid-containing vaccines are not known to be associated with [serious autoimmune adverse events],” which includes GBS. The plaintiff challenged the validity of the statement.

The issue of the weight and credibility of the testimony of the plaintiff’s experts regarding the studies is ultimately to be decided by a jury, the court concluded. The doctors relied upon epidemiological studies to demonstrate a causal connection between the vaccine and GBS. Dr. Posnanzy also used case reports, which can be included as one aspect of his opinion but cannot be solely what he based his opinion on.

The court found that the experts met the standards under Rule 702 and Daubert on general causation that the vaccine can cause GBS. The court therefore denied the motion to exclude the experts’ general causation opinions.

Specific causation opinion. Merck also challenged the specific opinion of Dr. Saperstein in which he concluded that Zostavax caused the plaintiff to develop GBS. Dr. Saperstein asserted that he performed a differential diagnosis to reach his conclusion. A differential diagnosis rules out obvious alternative explanations for the symptoms.

Dr. Saperstein opined that Zostavax caused the plaintiff’s GBS because of the short temporal proximity between when the plaintiff received the vaccine and when his symptoms began. He relied on his clinical experience and the fact that he has cared for over 100 patients with GBS.

Dr. Saperstein ruled out obvious known alternative causes, but he failed to rule out idiopathic, or unknown and unexplained causes. When unexplained causes are common, like with GBS, a differential diagnosis is insufficient when it does not eliminate these causes, the court stated. In his deposition, Dr. Saperstein stated that he did not rule out idiopathic causes of the plaintiff’s GBS.

Because the doctor did not have good grounds for his specific causation opinion because he could not rule out idiopathic causes, the court concluded that the doctor’s opinion did not meet the reliability standards of Rule 702 and Daubert. Accordingly, the court granted Merck’s motion to exclude Dr. Saperstein’s specific causation opinion.

The case is Nos. 2:18-cv-20020-HB; 18-md-2848.

Attorneys: Mark T. Sadaka (Sadaka Associates LLC) for Joseph Bockus. David J. Walz (Carlton Fields Jorden Burt PA) for Merck & Co., Inc. and Merck Sharp & Dohme Corp.

Companies: Merck & Co., Inc.; Merck Sharp & Dohme Corp.

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