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No significant increased risk of Bell's palsy was seen after the Pfizer-BioNTech mRNA vaccine, but risk did increase for people who received the CoronaVac (Sinovac Biotech) shot, a vaccine that uses an inactivated virus, an analysis in Hong Kong showed.

Compared with the background population, the age-standardized difference for Bell's palsy incidence was 41.5 (95% CI 11.7-71.4) per 100,000 person-years for CoronaVac and 17.0 (95% CI -6.6 to 40.6) per 100,000 person-years for Pfizer-BioNTech, reported Ian Chi Kei Wong, PhD, of University of Hong Kong, and colleagues.

For every 100,000 people vaccinated with CoronaVac, an additional 4.8 people may develop Bell's palsy, they wrote in . For every 100,000 people vaccinated with Pfizer-BioNTech, an additional two people may develop the condition.

"Our study suggests a small increased risk of Bell's palsy associated with CoronaVac vaccination," Wong said in a statement. "Nevertheless, Bell's palsy remains a rare, mostly temporary, adverse event."

"All evidence to date, from multiple studies, shows that the beneficial and protective effects of the inactivated COVID-19 vaccine far outweigh any risks," he added. "Ongoing surveillance through pharmacovigilance studies such as ours is important to calculate with increasing levels of confidence the risks of rare adverse events."

Facial paralysis can occur after viral infections, traumatic injury, cancer, or during pregnancy. Bell's palsy is a form of temporary paralysis or weakness on one side of the face that stems from dysfunction of cranial nerve VII. Symptoms appear suddenly over a few days and usually start to improve after a few weeks.

During the phase III trials of the Pfizer-BioNTech and Moderna mRNA vaccines, seven cases of facial paralysis or Bell's palsy were reported in the vaccine groups (7 of 35,654), and one case was seen in the placebo groups (1 of 35,611). A causal relationship was not established, but the FDA recommended that vaccine recipients be monitored.

In April, an analysis of pharmacovigilance data suggested mRNA vaccines did not have a higher reported rate of facial paralysis than other viral vaccines. In June, a case-control study in Israel found no association between recent vaccination with the Pfizer-BioNTech vaccine and risk of facial nerve palsy, and an analysis of medical records showed people infected with COVID-19 were more likely to develop Bell's palsy than people vaccinated against the virus.

The FDA and the U.K. Medicines and Healthcare products Regulatory Agency, among others, have argued that the observed frequency of facial palsy in vaccinated individuals was no higher than the expected background rate. On August 6, the Pfizer-BioNTech vaccine label to reflect very rare reports of Bell's palsy cases.

In their study, Wong and colleagues analyzed reports of Bell's palsy after people received one of two approved vaccines in Hong Kong: CoronaVac or Fosun-BioNTech (equivalent to Pfizer-BioNTech). The researchers used data from the Hong Kong drug regulatory authority pharmacovigilance system of adverse events reported by health professionals throughout the territory. Bell's palsy cases were included if they occurred within 42 days of the first or second vaccine dose.

Wong and colleagues also conducted a nested case-control study using territory-wide electronic health records of 298 clinically confirmed Bell's palsy cases and 1,181 matched controls.

From Feb. 23, 2021 to May 4, 2021, 451,939 people received the first dose of CoronaVac and 537,205 individuals received the first dose of Pfizer-BioNTech. A total of 28 confirmed cases of Bell's palsy were reported after the CoronaVac shot and 16 were reported after Pfizer-BioNTech.

The age-standardized incidence of clinically confirmed Bell's palsy was 66.9 cases per 100,000 person-years (95% CI 37.2-96.6) after CoronaVac vaccination and 42.8 per 100,000 person-years (95% CI 19.4-66.1) after Pfizer-BioNTech vaccination. The estimated background risk of Bell's palsy in Hong Kong in the previous decade was around 27 cases per 100,000 persons per year.

In the nested case-control analysis, adjusted ORs for Bell's palsy were 2.385 (95% CI 1.415-4.022, P=0.0011) for CoronaVac and 1.755 (95% CI 0.886-3.477, P=0.11) for Pfizer-BioNTech.

"Owing to the timing of vaccine rollout in Hong Kong, vaccination was notably less common in this population compared with the study done in Israel," observed Nicola Cirillo, DMD, PhD, of University of Melbourne in Australia, and Richard Doan, MD, of University of Toronto, in an .

In the Hong Kong study, only 84 (7.1%) people in the control population had been vaccinated versus 59.5% in the Israel study, which might have influenced the strength of the association, Cirillo and Doan noted. Data collected during the early days of vaccination in Hong Kong also may have introduced selection bias because only specific categories of workers and age groups were vaccinated at that time.

"These caveats are not trivial because the background incidence of Bell's palsy varies greatly with age and the vaccines are likely to have different safety profiles in different age groups," the editorialists pointed out.

"From a clinical, patient-oriented perspective, none of the studies published so far provide definitive evidence to inform the choice of a specific vaccine in individuals worldwide with a history of Bell's palsy," Cirillo and Doan added. "While waiting for conclusive evidence on vaccine-associated facial paralysis, one certainty remains: the benefit of getting vaccinated outweighs any possible risk."

The analysis has several limitations, Wong and colleagues acknowledged. Case surveillance was passive and relied on voluntary reporting from healthcare professionals; the extent of underreporting is unknown. In addition, socioeconomic status and education may have been important confounders, but these data were not available.

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