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Incidence of cerebral venous sinus thrombosis (CVST) remained a rare occurrence after the Johnson & Johnson COVID-19 adenovirus vector vaccine began being administered, researchers said.

The overall age- and sex-adjusted incidence of CVST was 2.34 per 100,000 person-years in Olmsted County, Minnesota, in 2001 to 2015, compared with a peak incidence of 8.65 per 100,000 person-years at 15 days postvaccination per the CDC Vaccine Adverse Event Reporting System (VAERS) database, according to a team led by Aneel Ashrani, MD, MS, of the Mayo Clinic in Rochester, Minnesota.

Those at highest risk of CVST after receiving the Johnson & Johnson product in early 2021 were women ages 30 to 39 (26.52 per 100,000 person-years) and women ages 40 to 49 (29.48 per 100,000 person-years) -- with women in general having a 5.1-fold higher risk after vaccination than they did before the COVID pandemic (13.01 vs 2.53 per 100,000 person-years, P<0.001).

The study authors maintained that the "absolute CVST risk was still low" for these women. "The reason that women had a higher incidence of postvaccination CVST is unclear; concomitant CVST risk factors or autoantibody production might have been involved," they noted in .

"In this population-based cohort study, we found that the CVST incidence rate 15 days after Ad26.COV2.S vaccination was significantly higher than the prepandemic rate. However, the higher rate of this rare adverse effect must be considered in the context of the effectiveness of the vaccine in preventing COVID-19," Ashrani's group urged.

Other research has found that severe COVID-19 is associated with more cases of CVST compared with vaccination by an order of magnitude.

Another previous report detailed six cases of CVST with thrombocytopenia -- all in women and all within 6 to 13 days of vaccination -- out of 6.85 million people who had received the Johnson & Johnson vaccine since.

That report led to a brief pause in use of the adenovirus vector vaccine in April.

For the present study, Ashrani and colleagues sifted through Olmsted County's prepandemic CVST cases. Of the 39 cases found, 29 were associated with a predisposing venous thromboembolism risk factor, such as infection, active cancer, and recent oral contraceptive use. Median age at CVST diagnosis was 41 years, and 56.4% were women.

In comparison, there were 38 CVST events backed by sufficient details or imaging in VAERS reports from people who received the Johnson & Johnson vaccine from February 28 to May 7 of this year. Of those cases, 27 occurred in women, and the median age was 45 years.

The investigators noted the potential for ascertainment bias in their study, since they included only "objectively diagnosed" CVST cases. They also cautioned that reporting to VAERS is voluntary.

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