Earlier this year the US Centres for Disease Control and Prevention (CDC) reported a signal for ischaemic stroke in those aged 65 and older who had received a certain covid vaccine.
The researchers behind a new study titled “BA.1 Bivalent COVID-19 Vaccine Use and Stroke in England” believe that this increase in risk may relate to concurrent administration of high-dose or adjuvanted influenza vaccine.
This signal made by the CDC specifically relates to the Pfizer-BioNTech, bivalent (BA.4/BA.5) vaccination. However, no other assessments in the US have validated this signal, furthermore, no signal was observed with the Moderna vaccine.
The study
August 2022, in the UK saw another round of covid booster vaccinations campaigns for those aged 50 years or older. During this campaign the Pfizer-BioNTech and Moderna vaccinations were both used, however, they contained the BA.1 rather than BA.4/BA.5
This study investigates the association between these vaccines and ischaemic stroke and the effect of simultaneous influenza vaccination on the association.
NHS hospital admissions in England ranging from September 2022 to December 2022 for ischaemic stroke (including mini stroke) or haemorrhagic stoke (for comparison) in individuals aged 50 or older on August 31, 2022, were linked to the National Immunisation Management System via their NHS number.
During the study period, 14.6 million doses of the Moderna vaccine were given to those aged 50 years and older and there was 6882 cases of ischaemic stroke and 1510 of haemorrhagic stroke after the booster. Of these patients, 982 (11.7 per cent) had received simultaneous influenza vaccine; for 822 (94.6 per cent) of those aged 65 years and older, the product was adjuvanted.
Table 2 displays the number of cases In the 1 to 21 day post vaccination risk period and the control period for each analysis along with the relative incidence (RI) estimates. The average length of the control period was 51 days for those receiving the Moderna vaccine and 29 days for those receiving the Pfizer-BioNTech.
For those aged 65 and older and received the Pfizer-BioNTech vaccine, the mean person-days in the risk interval were 20.9 vs 26.3 in the control interval (RI for ischaemic stroke, 0.90; 95% CI, 0.76 – 1.05) For patients aged 65 years and older and also receiving influenza vaccine, the mean person-days in the risk interval were 21.0 vs 25.4 in the control interval (RI for ischaemic stroke, 0.79; 95% CI, 0.50-1.23).
Results were similar for individuals aged 50 years and older, receiving the Moderna vaccine, or with haemorrhagic stroke. The lower bound of the 95 per cent CI for all RI estimates was below 1.
Discussion
The researchers state that their research shows no evidence of an increased risk of stroke in the 21 days immediately after vaccination with either of the 2 Moderna COVID-19 bivalent BA.1 vaccines in England, with similar results for ischaemic and haemorrhagic stroke and for the subset aged 65 years and older given influenza vaccine on the same day as the bivalent COVID-19 vaccine.
For Ischaemic stroke, the upper bounds of CIs for the RI were all below the point estimate reported by the CDC of relative risk of 1.47.
