COVID-19 May Leave a Lasting Mark on Allergy Risk, Massive Japanese Study Finds

A sweeping analysis of Japan’s national health insurance records has uncovered a striking pattern in the aftermath of SARS-CoV-2 infection: people who had COVID-19 were significantly more likely to receive a new prescription for allergy medication than closely matched individuals who had never been infected. The finding, drawn from one of the largest cohort studies yet conducted on the long-term immunological consequences of the virus, adds a new dimension to the growing catalogue of post-acute sequelae associated with COVID-19 and suggests that the pandemic’s shadow may extend well beyond the respiratory symptoms that dominated the early years of the crisis.

The study, led by Daisuke Miyamori, Shuhei Yoshida, and Masanori Ito of the Department of General Internal Medicine at Hiroshima University Hospital, was published in BMC Public Health. The researchers harnessed the National Database of Health Insurance Claims and Specific Health Checkups of Japan, known as the NDB, an extraordinarily comprehensive repository that captures prescription activity across virtually the entire Japanese population. Because Japan maintains a universal insurance system, the database offers an unusually complete picture of who receives which medications, when, and in what clinical context, making it a powerful instrument for detecting subtle shifts in disease incidence at the population scale.

The methodological core of the investigation was a matched cohort design, one of the most rigorous observational frameworks available for questions of this kind. The team identified 778,362 individuals who had been diagnosed with COVID-19 and then constructed a comparison group of uninfected patients matched on key characteristics, ensuring that the two groups were as similar as possible apart from their infection status. Matching on factors such as age, sex, and baseline health status is critical in pharmacoepidemiology because it reduces confounding, the persistent threat that an apparent association between infection and prescription rates might actually reflect underlying differences between the groups rather than a genuine biological effect of the virus.

Once the cohorts were assembled, the researchers tracked new prescriptions for four classes of allergy-related medications: H1 antihistamines, which block the histamine receptors that drive itching, sneezing, and urticaria; leukotriene receptor antagonists, which target a different inflammatory pathway central to asthma and allergic rhinitis; ophthalmic drops for allergic eye disease; and nasal sprays for rhinitis. Crucially, the team counted only new prescriptions, meaning patients who had already been receiving these drugs before their infection or matching date were excluded from the outcome analysis. This distinction matters enormously. A rise in refills among existing allergy sufferers would tell a very different story from a rise in people who had never needed allergy medication before suddenly requiring it, and only the latter points toward genuinely new allergic disease.

Over a median follow-up of seven months, the signal was clear and statistically robust. The incidence rate ratio for new allergy drug prescriptions among the COVID-19 group, compared with controls, was 1.21, with a 95 percent confidence interval of 1.20 to 1.23. In practical terms, for every million person-months of follow-up, the infected group accumulated roughly 1,528 more new prescriptions than the uninfected group, a figure the researchers report as an incidence rate difference with a confidence interval of 1,492 to 1,564. A 21 percent relative elevation may sound modest, but applied across the hundreds of millions of infections that occurred worldwide during the pandemic, it translates into a substantial burden of newly medicated allergic disease.

Perhaps the most consequential findings emerged from the subgroup analyses, which probed whether the effect of COVID-19 was distributed evenly across the population or concentrated in particular groups. The answer was emphatic: the elevation was markedly stronger among older adults and among patients with significant pre-existing illness. For individuals aged 65 and above, the incidence rate ratio climbed to 1.33, with a confidence interval of 1.31 to 1.35, while for patients scoring 4 or higher on the Charlson Comorbidity Index, a validated composite measure that weights conditions such as diabetes, chronic kidney disease, and heart failure, the ratio reached 1.35. These are not marginal statistical artifacts; the confidence intervals sit comfortably above the null value of 1.0, indicating that the excess risk in these vulnerable subgroups is unlikely to be a chance finding.

The sensitivity analyses extended the picture into the temporal domain, addressing the question that haunts much of the long COVID literature: does the effect fade, or does it persist? When the researchers examined follow-up periods extending beyond one year, the elevated prescription rates remained detectable. Among those aged 65 and older, the incidence rate ratio was still 1.14 at more than a year post-infection, and among patients with Charlson Comorbidity Index scores of 4 or greater it remained 1.17. The attenuation relative to the overall follow-up window suggests some degree of normalization over time, but the persistence of a measurable excess a full year after infection argues against the phenomenon being a transient artifact of post-viral irritation or short-lived diagnostic attention.

The biological plausibility of these findings rests on well-characterized immunological mechanisms. SARS-CoV-2 infection is known to perturb immune regulation in ways that can outlast the acute illness. Studies have documented shifts in T cell populations, dysregulation of type 2 immune pathways that govern allergic responses, and elevations in IgE, the antibody class central to allergic sensitization, following viral infections. Viral respiratory infections in general have long been implicated in the initiation and exacerbation of allergic airway disease, with epithelial damage and barrier disruption in the nasal and bronchial mucosa creating conditions favorable to allergen penetration and chronic inflammation. A SARS-CoV-2-driven tilt of the immune system toward type 2 skewing would provide a coherent mechanistic account for increased demand for antihistamines, leukotriene modifiers, and topical anti-inflammatory preparations.

The authors are careful to frame their conclusions within the limits of an observational, claims-based design. Prescription records are a proxy for diagnosed allergic conditions, not a direct measurement of disease, and prescribing behavior can be influenced by factors other than underlying pathology, including heightened patient vigilance after a serious illness and increased contact with the health system. The median follow-up of seven months, while sufficient to detect the signal, leaves longer horizons incompletely characterized for much of the cohort. Residual confounding cannot be entirely excluded in any non-randomized comparison, however meticulous the matching. Nevertheless, the consistency of the effect across four distinct drug classes, the dose-response-like gradient across age and comorbidity strata, and the persistence in sensitivity analyses collectively strengthen the case that the association reflects a real clinical phenomenon rather than a statistical mirage.

The public health implications are considerable. If COVID-19 genuinely accelerates the onset of allergic disease, health systems should anticipate elevated demand for allergy care in the years following waves of infection, particularly among older adults and patients with multimorbidity, the very groups already bearing the heaviest burden of post-acute sequelae. The researchers emphasize the need for continued monitoring and for personalized post-COVID-19 support, a recommendation that aligns with a broader shift in the field toward viewing SARS-CoV-2 not merely as an acute respiratory pathogen but as a systemic insult with heterogeneous, organ-spanning, and sometimes durable consequences. For clinicians, the study suggests a lowered threshold for evaluating new allergic symptoms in post-COVID patients; for researchers, it opens a rich vein of questions about mechanism, duration, and reversibility. As the pandemic recedes from daily attention, studies of this scale and rigor serve as a reminder that its epidemiological aftermath is still being written, one prescription record at a time.

Subject of Research: The association between COVID-19 infection and subsequent new allergy medication prescriptions in Japan

Article Title: COVID-19 and subsequent new allergy drug prescriptions: a matched cohort study in Japan

Article References: COVID-19 and subsequent new allergy drug prescriptions: a matched cohort study in Japan. (n.d.). https://doi.org/10.1186/s12889-026-29757-0

Image Credits: AI Generated

DOI: 10.1186/s12889-026-29757-0

Keywords: COVID-19, allergy, SARS-CoV-2, antihistamines, leukotriene receptor antagonists, matched cohort study, Japan, national insurance claims database, post-acute sequelae, long COVID, allergic rhinitis, public health