The Covid vaccination rollout represents the largest experiment the world has ever run, with hundreds of millions of people getting inoculated. It has enabled countries with high vaccination rates to unlock their economies and restore freedoms to their citizens. The benefits of getting vaccinated far outweigh any potential risks but studies continue to monitor for any adverse reactions as efforts are scaled up across the globe.
In the new research, published in The Lancet Infectious Diseases journal, researchers analysed cases of the condition related to the two approved vaccines in Hong Kong – CoronaVac and the Pfizer jab.
Bell’s palsy cases were included in the analysis if they occurred within 42 days of a first or second vaccine dose, within the timeframe of the study.
Researchers also conducted a case-control study using territory-wide electronic health records databases including 298 Bell’s palsy cases and 1,181 matched controls.
Between February 23 and May 4 this year, 28 clinically confirmed cases of Bell’s palsy were identified among the 451,939 people who received at least a first dose of CoronaVac.
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Sixteen cases were identified among the 537,205 individuals who received at least a first dose of the Pfizer vaccine.
Experts analysed data from 2010-20, and estimated the background risk of Bell’s palsy in Hong Kong – around 27 cases per 100,000 people per year.
Global estimates range from 15 to 30 cases.
The nested case-control study – whereby cases of a disease that occur in a defined cohort are identified – found that receiving CoronaVac was associated with 2.4 times increased risk of Bell’s palsy, whereas the Pfizer jab was not associated with a significantly increased risk.
Lead author Professor Ian Chi Kei Wong, from the University of Hong Kong, said: “Our study suggests a small increased risk of Bell’s palsy associated with CoronaVac vaccination.
“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.
“Ongoing surveillance, through pharmacovigilance studies such as ours, are important to calculate with increasing levels of confidence the risks of rare adverse events.”
The scientists could not conclude a causal relationship between Bell’s palsy and vaccination in any individual cases from this study, and the mechanism by which vaccination can, in very rare instances, lead to Bell’s palsy remains unclear.
It is important to note that other studies have identified rare cases of Bell’s palsy after other inactivated vaccines, such as for influenza.
What’s more, the study was limited to patients with a new diagnosis of Bell’s palsy in Hong Kong, so further studies including patients with a history of Bell’s palsy and patients in other regions should be done to confirm the findings.
Further analyses are also needed to understand whether the risk varies by sex or age, the experts said.