In this January 2026 update, Dr. Roger Seheult of MedCram examines a striking and predictable pattern: influenza cases in the United States have already peaked and are declining, and this peak occurs almost every year about one to three weeks after the shortest day of the year. Data reported to the Centers for Disease Control and Prevention show this seasonal crest clearly, with influenza test positivity rising in late December and early January before trending downward. The same pattern is seen in the United Kingdom, where multiple age groups peak around the same time, though older adults may experience a more prolonged course.
Monitoring patterns
What makes this pattern particularly interesting is that it is not limited to influenza and pneumonia. Seasonal variation in mortality affects many conditions, including heart disease. In fact, the winter increase in cardiac deaths exceeds the total number of deaths attributed to influenza and pneumonia during flu season. This predictable annual rhythm raises an important question: what is driving it? Common explanations include colder temperatures, lower humidity, or increased indoor gatherings during the holidays. However, when examined more closely, these explanations fall short.
What about holidays?
If holiday gatherings were the primary driver, we would expect similar timing in countries regardless of hemisphere. Yet in Australia, influenza season peaks approximately six months out of phase with the United States, occurring one to three weeks after their shortest day of the year in June. Christmas and New Year’s celebrations there occur during summer, when influenza activity is minimal. Temperature alone also fails to explain the pattern. For example, winter temperatures in Sydney are relatively mild, with highs in the mid-60s Fahrenheit and lows in the upper 40s, hardly comparable to harsh winters in the northeastern United States. Meanwhile, equatorial regions such as Singapore do not show the same pronounced seasonal pattern, consistent with the fact that day length and solar radiation remain relatively stable year-round near the equator.
What about latitude?
The same latitude-dependent phenomenon was observed during the COVID-19 pandemic. A January 2021 analysis of Europe’s autumn 2020 surge found no meaningful correlation between case increases and either temperature or humidity. However, there was a striking relationship with latitude and, more specifically, declining ultraviolet B radiation. The timing of COVID-19 surges moved progressively from northern countries like Finland down toward southern regions such as Greece, closely tracking the point at which ultraviolet B levels dropped below approximately 34 percent of equatorial intensity. The correlation coefficient was extraordinarily high, suggesting a strong and consistent association across countries with different healthcare systems and political structures.
Additional research from the Harvard Kennedy School examined solar radiation data alongside influenza trends, including during the 2009 H1N1 pandemic. That year was unique because influenza surged in the spring and early summer rather than during the typical winter months, allowing researchers to separate temperature effects from sunlight exposure. Their findings indicated that higher levels of solar radiation were strongly protective against influenza. Similar associations have been observed with COVID-19 mortality in the United States, England, and Italy, with higher solar radiation correlating with reduced mortality independent of vitamin D pathways. This suggests that mechanisms beyond vitamin D, potentially including infrared light and other photobiological effects, may contribute to immune resilience.
Global analyses further support the latitude connection. In countries where more than half of the population is overweight, there appears to be a notable association between COVID-19 mortality and distance from the equator, reinforcing the possibility that reduced sunlight exposure may compound metabolic vulnerability. Taken together, these findings point toward sunlight—and not simply temperature—as a major environmental variable influencing infectious disease seasonality and overall mortality.
What about NAC?
As influenza cases decline from their winter peak in the Northern Hemisphere, increasing safe sunlight exposure may be one practical strategy to support immune health. In addition to standard preventive measures such as vaccination and hand hygiene, there is also evidence supporting certain adjunctive therapies. A randomized, placebo-controlled, multicenter trial published in 1997 involving 262 participants found that N-acetylcysteine at a dose of 600 mg twice daily for six months significantly reduced influenza symptom severity. Among those who contracted influenza, only 25 percent of participants in the treatment group developed symptomatic illness compared to 79 percent in the placebo group, representing a substantial absolute risk reduction.
At MedCram, the emphasis is not only on traditional medical education but also on evidence-based, non-pharmacologic interventions that may not receive widespread attention. While discussions of flu shots and hygiene are common, conversations about sunlight exposure and randomized trials involving over-the-counter compounds such as N-acetylcysteine are less frequent, despite compelling data. Influenza continues to cause tens of thousands of deaths annually in the United States and worldwide. Understanding the role of seasonal sunlight variation may offer a broader perspective on why these predictable winter surges occur and what practical steps individuals can take to mitigate risk.
LINKS / REFERENCES:
Attenuation of influenza-like symptomatology and improvement of cell-mediated immunity with long-term N-acetylcysteine treatment (The European respiratory journal) | https://pubmed.ncbi.nlm.nih.gov/9230243/
Weekly US Influenza Surveillance Report: Key Updates for Week 53, ending January 3, 2026 (CDC) | https://www.cdc.gov/fluview/surveilla…
National flu and COVID-19 surveillance report: 8 January 2026 (UK) | https://www.gov.uk/government/statist…
Influenza-associated mortality in Australia, 2010 through 2019: High modelled estimates in 2017 (Vaccine) | https://www.sciencedirect.com/science…
Autumn COVID-19 surge dates in Europe correlated to latitudes, not to temperature-humidity, pointing to vitamin D as contributing factor (Scientific reports) | https://pmc.ncbi.nlm.nih.gov/articles…
Sunlight and Protection Against Influenza (Harvard) | https://www.hks.harvard.edu/publicati…
Ultraviolet A radiation and COVID-19 deaths in the USA with replication studies in England and Italy (The British journal of dermatology) | https://pubmed.ncbi.nlm.nih.gov/33834…
A geographical approach to the development of hypotheses relating to Covid-19 death rates (Melatonin Research) | https://www.researchgate.net/publicat…