Sunlight, Seasons and Survival: Vitamin D Fails to Explain Timing Effects in Immunotherapy

One of the most persistent puzzles in modern cancer immunotherapy may have just lost its most popular explanation. A large retrospective study from The Christie NHS Foundation Trust in Manchester, one of the United Kingdom’s largest cancer centres, has examined whether the season in which patients begin immune checkpoint inhibitor treatment influences how long they live, and whether shifting vitamin D levels across the year could account for any such effect. The findings, published in Cancer Immunology, Immunotherapy, deliver a nuanced verdict: across a pan-cancer cohort of 4,329 patients, the broad seasonal timing of treatment initiation showed no consistent association with overall survival, even though vitamin D status itself was strongly linked to outcomes and varied dramatically with the calendar.

Immune checkpoint inhibitors, including antibodies that block PD-1, PD-L1 and CTLA-4, have transformed the treatment of melanoma, lung cancer and many other malignancies by releasing the molecular brakes on T cells. Yet responses are unpredictable, and researchers have long searched for modifiable factors that might explain why some patients derive extraordinary benefit while others progress rapidly. Among the candidates, vitamin D has attracted particular attention. The secosteroid hormone regulates innate and adaptive immunity, influences myeloid and lymphocyte function, and has been associated in multiple studies with better outcomes for patients receiving immunotherapy. Because vitamin D status in countries at high latitudes follows a pronounced seasonal rhythm, driven by ultraviolet B exposure and cutaneous synthesis, some investigators hypothesised that the time of year a patient starts therapy could subtly shape the immune environment in which checkpoint blockade takes effect.

Previous reports in melanoma had fuelled this idea, with conflicting evidence about whether summer initiation of ipilimumab, for example, was associated with longer survival than winter initiation. To test the hypothesis at scale, the Manchester team, led by Oliver Kennedy and Paul Lorigan, conducted a single-site retrospective review of patients treated with checkpoint inhibitors between 2018 and 2023. They categorised patients by season of treatment initiation using two complementary definitions: the broader split of October to March versus April to September, aligned with the UK’s pattern of vitamin D insufficiency, and a narrower comparison of December to February versus June to August, capturing the extremes of winter and summer. Vitamin D status was classified using standard laboratory thresholds, with sufficiency defined as serum 25-hydroxyvitamin D above 50 nmol/L, insufficiency between 25 and 50 nmol/L, and deficiency below 25 nmol/L.

The statistical machinery applied to the data was conventional but rigorous. Kaplan-Meier curves provided estimates of median overall survival and one-year survival, while Cox proportional hazards models generated adjusted hazard ratios that accounted for potential confounders. Among the 4,329 patients analysed, 2,191 deaths were recorded, giving the study substantial statistical power. The headline result was strikingly null: patients who began checkpoint inhibitors between April and September fared no better than those who started between October and March, with an adjusted hazard ratio of exactly 1.00. One-year survival was virtually identical in the two groups, at 64.9 percent versus 65.0 percent, and median overall survival differed by less than a month, at 21.3 versus 21.8 months.

Beneath that overall null result, however, the data revealed intriguing signals at the level of specific regimens and diseases. When the analysis was restricted to the narrower seasonal comparison, initiation in June to August was associated with longer overall survival than initiation in December to February for patients receiving the combination of nivolumab and ipilimumab, with an adjusted hazard ratio of 0.64, and for patients with small cell lung cancer, with an adjusted hazard ratio of 0.52. No such association was observed for other checkpoint inhibitor regimens or other cancer types. These subgroup findings suggest that if seasonal biology matters at all, its influence may be confined to particular treatment combinations or tumour contexts, a pattern that the authors caution requires confirmation in independent cohorts before any clinical conclusions are drawn.

The vitamin D measurements added a second layer to the story. Among the 1,206 patients whose vitamin D status was available, deficiency was far from rare, and it followed the expected seasonal gradient with remarkable fidelity. The proportion of deficient patients peaked at 28.4 percent in March, the tail end of the British winter when cutaneous synthesis has been minimal for months, and fell to just 3.5 percent in August, after a summer of sun exposure. That nearly tenfold seasonal swing in deficiency prevalence confirms that the study population experienced the kind of vitamin D variation that, in theory, could modulate immune function and thereby influence immunotherapy outcomes.

And vitamin D status did matter for survival. After adjustment, patients with insufficient vitamin D had a hazard ratio of 1.64 for death compared with sufficient patients, while those who were frankly deficient had a hazard ratio of 2.38. In other words, deficient patients were more than twice as likely to die over the follow-up period as those with adequate levels, a magnitude of association that rivals many established prognostic factors in oncology. Yet when the researchers examined the subgroup of vitamin D-sufficient patients, the season of treatment initiation showed no association with survival at all, undermining the hypothesis that seasonal vitamin D fluctuations are the mechanism driving any apparent seasonal survival effects.

The authors offer two possible interpretations of this pattern, and both carry important implications. The first is that the seasonal variation in vitamin D observed in the cohort, while real, was simply not large enough to alter the effectiveness of checkpoint inhibitors in a way that translates into measurable survival differences. The second, more sobering possibility is that the well-documented association between vitamin D status and immunotherapy outcomes may itself be confounded. Vitamin D levels correlate with general health, mobility, outdoor activity, nutritional status and disease burden, all of which independently influence survival. Patients who are well enough to spend time outdoors in the summer sun may simply be healthier in ways that a blood test cannot capture. If that is the case, vitamin D could be a marker of prognosis rather than a mediator of immunotherapy response, and trials of supplementation would be unlikely to improve outcomes.

Distinguishing between these explanations is precisely what the field now needs. Randomised trials of vitamin D supplementation in patients receiving checkpoint inhibitors are the cleanest way to test causality, and the new findings sharpen the rationale for such studies by showing that observational season-based approaches cannot substitute for them. The regimen-specific signals, particularly the apparent summer advantage for nivolumab plus ipilimumab and for small cell lung cancer, also merit dedicated investigation, since combination CTLA-4 and PD-1 blockade engages immune biology in ways that differ from single-agent therapy, and small cell lung cancer has a distinct immunological and epidemiological profile from most other tumours treated with checkpoint inhibitors.

For patients and clinicians, the practical message is measured. There is no evidence from this study that the timing of the year should influence when immunotherapy is started; cancer treatment should proceed on clinical grounds without regard to the calendar. At the same time, the strong association between vitamin D deficiency and poorer survival reinforces existing guidance that patients with cancer should have their vitamin D status checked and corrected where appropriate, in line with standard nutritional care. What the Manchester study removes is a seductive but unsupported idea: that Britain’s long winters might be quietly sabotaging immunotherapy. The seasons, it turns out, shape our vitamin D levels profoundly, but whether that biology shapes cancer survival remains an open and increasingly well-defined question.

Subject of Research: Seasonal variation in vitamin D status and overall survival in patients receiving immune checkpoint inhibitors

Article Title: Seasonal variations in overall survival and vitamin D status in a UK pan-cancer cohort receiving checkpoint inhibitors

Article References: Seasonal variations in overall survival and vitamin D status in a UK pan-cancer cohort receiving checkpoint inhibitors. (n.d.). https://doi.org/10.1007/s00262-026-04575-w

Image Credits: AI Generated

DOI: 10.1007/s00262-026-04575-w

Keywords: immune checkpoint inhibitors, vitamin D, overall survival, seasonality, immunotherapy, melanoma, lung cancer, nivolumab, ipilimumab, pan-cancer cohort, retrospective study, cancer outcomes