JN.1 COVID-19 Booster Safety Confirmed in Large Study: Daily Dose

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Patient Care brings primary care clinicians a lot of medical news every day—it’s easy to miss an important study. The Daily Dose provides a concise summary of one of the website's leading stories you may not have seen.

On August 13, 2025, we reported on a study published in JAMA Network Open that examined the association between vaccination with an updated mRNA COVID-19 vaccine targeting the Omicron JN.1 lineage and the risk of 29 serious adverse events.

The study

Researchers conducted a nationwide Danish cohort study of adults with a mean age of approximately 67 years who received the 2024–2025 seasonal booster. Follow-up ran from May 1, 2024, to March 31, 2025.

Investigators evaluated 29 predefined adverse events of special interest, adapted from international safety monitoring lists, using first hospital contact for a relevant diagnosis as the event definition. The team compared rates during the 28-day risk window after JN.1 vaccination with rates during a reference period (prevaccination or more than 43 days postdose) using Poisson regression with multivariable adjustment for age, sex, region, vaccination priority group, calendar time, and comorbidity count.

The findings

The study cohort included 1 585 883 individuals (mean age 66.8 years; 54.4% women). Of this group, 1 012 400 (mean age 73.5 years) received the JN.1-containing booster.

Selected adjusted IRRs (95% CI) included:

• Ischemic cardiac events: 0.84 (0.76–0.94)

• Intracranial bleeding: 0.92 (0.76–1.13)

• Myocarditis: 1.12 (0.41–3.10)

• Guillain-Barré syndrome: 0.42 (0.20–0.89)

• Pulmonary embolism: 0.75 (0.62–0.90)

For 19 of the 29 outcomes, the upper CI bound was inconsistent with even a moderate to large relative risk increase (ie, greater than 1.5), a finding providing additional reassurance, authors wrote. They also reported that rare events such as transverse myelitis were too infrequent to estimate risk ratios reliably.

Authors' comments

"In this nationwide cohort study, no increased risk of 29 adverse events was observed after vaccination with the updated COVID-19 mRNA vaccine containing the SARS-CoV-2 Omicron JN.1 lineage in approximately 1 million adults. Limitations include that residual confounding and health care use bias cannot be excluded even with the use of within-individual comparisons."

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