Hong Kong study: Benefits of Covid-19 vaccines far outweigh rare risk of Bell’s palsy

The study, published online in The Lancet Infectious Diseases journal, finds that for every 1,00,000 people vaccinated with CoronaVac, an additional 4.8 people may develop the condition.

The first large-scale population-based study on the association between Covid-19 vaccines and Bell’s palsy confirms that the beneficial and protective effects of the vaccines far outweigh the risk of this rare adverse event, which had been reported in clinical trials of vaccines.

Bell’s palsy is the sudden onset of one-sided facial paralysis. In the majority of cases (70 per cent), the condition resolves itself within six months without treatment and the chance of recovery is even higher (90 per cent) if patients receive early treatment with corticosteroids.

In this study, researchers analysed cases of Bell’s palsy related to the two approved vaccines in Hong Kong — CoronaVac (equivalent to Sinovac) and BNT162b2 (equivalent to Pfizer). The study, published online today (Tuesday) in The Lancet Infectious Diseases journal, finds that for every 1,00,000 people vaccinated with CoronaVac, an additional 4.8 people may develop the condition. For BNT162b2, an additional 2 cases per 1,00,000 vaccinated people. Further studies with a sufficient sample size are needed to evaluate the association between Bell’s palsy and BNT162b2.

Current prescribing information for CoronaVac, an inactivated virus vaccine, does not list Bell’s palsy as a rare adverse event, but based on the findings from the study, approval has been obtained to include information on the potential association.

“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,” says lead author Professor Ian Chi Kei Wong, The University of Hong Kong.

The study used data from the Hong Kong drug regulatory authority pharmacovigilance system, which included reports of adverse events logged by health professionals throughout the territory. Cases of Bell’s palsy were included in the analysis if they occurred within 42 days of the first or second vaccine dose, within the timeframe of the study. They also conducted a nested case-control study using territory-wide electronic health record database including 298 Bell’s palsy cases and 1181 matched controls.

Between February 23rd, 2021 and May 4th, 2021, 28 clinically confirmed cases of Bell’s palsy were identified among the 4,51,939 individuals who received at least one dose of CoronaVac (equivalent to 3.61 cases per 1,00,000 doses) and 16 cases were identified among the 5,37,205 individuals who received at least one dose of BNT162b2 (equivalent to 2.04 cases per 1,00,000 doses).

By analysing data from 2010-2020, the researchers estimated the background risk of Bell’s palsy in Hong Kong – around 27 cases per 100,000 people, per year. Global estimates range from 15-30 cases per 100,000 people, per year. The nested case-control study found that receiving CoronaVac was associated with 2.4 times increased risk of Bell’s palsy whereas receiving BNT162b2 was not associated with a significantly increased risk.

“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,” says lead author Professor Ian Chi Kei Wong, The University of Hong Kong.

The authors note that they cannot conclude a causal relationship between Bell’s palsy and vaccination in any individual cases from this study, and that the mechanism by which vaccination can – in very rare instances – lead to Bell’s palsy remains unclear.

A small number of cases of Bell’s palsy have been reported in clinical trials of mRNA Covid-19 vaccines, but so far, analyses of the association have come to conflicting conclusions, the Lancet study has said. In the USA, the Federal Drug Administration (FDA) did not consider there to be a clear causal association for the two mRNA vaccines BNT162b2 (Pfizer-BioNTech) and the mRNA-1273 (Moderna) vaccines but recommended ongoing surveillance. Acute partial facial paralysis is reported as a rare side effect of both vaccines by the European Medicines Agency.

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