COVID-19 Seasonality Explained

Grass pollen seasonal COVID-19

COVID-19 and Flu are both seasonal, and solar radiation, seasonal allergens and seasonal behavior appear to be jointly responsible for that. This is in short, the outcome of our research during 2020-2021 in co-operation with researchers from the Open University Netherlands, Jeroen Bosch Medical Center, Erasmus University Medical Center and Leiden University Medical Center.

See our latest paper Environmental factors and mobility predict COVID-19 seasonality.

June 7, 2021: OneHealth publishes our 3rd article which shows that “Seasonal patterns COVID-19 and Flu-Like Illnesses comparable” (preprint version of March 2021) for a country in the temperate climate zone. The seasonality of Flu is a well-accepted, non-controversial fact. But, regarding COVID-19 it was mere speculation, often not included in the predictive models of our governments. The confirmation of COVID-19 seasonality is therefore very impactful: we can again start planning to lift lockdown measures (earlier) during Spring. And lift these when helped or natural immunity levels protect us well enough against serious outcomes.

March 10, 2021: interview in about whether pollen reduces or contributes to COVID-19 infection rate, and the serious methodological limitations of a recent German study.

Seasonal allergens (pollen) explain Flu and COVID-19 season

Feb 16, 2021: An independent Chicago study confirms our “seasonal allergens” findings. This study ads mold spores to the bioaerosol count and provides some additional pathophysiological explanations:

Feb 10, 2021: Science of the Total Environment (STE) publishes our paper “Can pollen explain the seasonality of flu-like incidence?“. STE is a respected, peer-reviewed journal of Elsevier Science with an impact factor of 6.5 (2019). The impact factor is based on the average number of citations per year.

In this second study, we explain the seasonality of flu-like incidence including COVID-19. In summary, the study looks at recent medical explanations of the protective effects of seasonal allergens and allergies in relation to influenza and COVID-19. Further, we tested associations between seasonal allergens and flu-like incidence. Finally, we controlled outcomes for meteorological variables.

July 20, 2020: preliminary study is published after peer-review in which I identified pollen as an inhibitor of flu-like incidence.

Media coverage pre-print

Even after its publication, the study is still kept available as a preprint on medRxiv. Preprint servers are a wonderful tool to share new insights and collect feedback during the peer-review process of a journal.

At first, the popular medical website News Medical picked up the pre-print. Followed by the French, and Thailand Medical News, covering our findings extensively as well.

Hayfever countries like the findings

Our findings were especially well covered in typical hayfever countries. Yahoo Japan mentions the outcomes, based on an item in the South-Korean newspaper 中央日報 (Central Daily News).

In Russia, Naked Science (RU) summarizes and LIFE (RU) and Izvestia (RU) discuss the outcomes. This leads to coverage by Shafaaq (Iraq), and Eg24 (Egypt). The South China Morning Post says “Pollen could play a role in reducing [the] spread of coronavirus, Dutch study finds.” In Chinese, there are many items. For example, in Asia News,,,,, etc.

Indeed, the appearance of a large amount of pollen in the air can lead to the binding of micro-droplets containing the virus, which slows its spread.

Oleg Batishchev, associate professor of the Department of Biophysics at the Moscow Institute of Physics and Technology. Source: newspaper Izvestia (RU).

Regional Dutch newspapers on October 16, 2020, published an interview about the implications for the second wave of COVID-19. I expect it to be longer than the first wave and end around week 10 of 2021 (+/- 5 weeks). The end will be similarly abrupt as the ending of the first wave. In the Netherlands, the government follows a flatten-the-curve policy. Not a hard containment policy like in East Asia and the Pacific.

The first newspaper to pick the story up is FD. Next follows an interview in De Telegraaf, the most read Dutch newspaper, provoking extensive online discussions. Someone posted an English translation of it. Also, RTL News (NL) had an item (@2m10s) about it as well. It’s clear that COVID-19, hay fever, and multi-cycle pandemics are big subjects, as is the riddle of flu-like-seasonality. We identified a key predictor.

Founder and CEO of Icecat NV. Investor. Ph.D.

  • commented on June 8, 2020 by Raan

    Indoor confinement adequately explains flu seasonality does it not?

    • Thanks an excellent question.

      The standard misconception is that flu-season is not overlapping Summer, when people are more outdoors.

      The simple answer is therefore: No, being confined indoors is not a good explanation. Flu-season is not absent in “Summer”, as is the popular belief, but statistically it starts every year as early as half August, thus in the middle of the Summer in the Northern Hemisphere, when people are still a lot outdoors. The Summer technically ends September 22.

      The more complex answer is: In general, the science consensus is that meteorological variables and seasonal behavior don’t sufficiently explain flu-like seasonality. Also behavior associated to the weather can’t provide satisfactory explanations for the start and end of flu-like seasonality. Below, still a bit more in detail.

      You might notice in the 2nd paper that, for this reason, we especially discuss the beginning and end of flu-season. The early start of flu-like season (reproduction numbers structurally >1) is half August (wk 33 +/- 1 week) and it can have average temperatures around 17 oC. This is in NL (and many Northern Hemisphere countries) the hottest, most sunny month, on average. People are then still going out, enjoying terraces, having a good time, sometimes on holidays.

      By contrast the ending of flu-like season (reproduction number structurally < 1) around wk 10 (+/- 5 weeks) typically shows far lower temperatures, sometimes as low as 0 oC. People can be out on nice days, but are still a lot of time inside when it's less sunny. Although, in general, we all can agree that behavior - hygiene, social distancing - is important to contain an epidemic/pandemic, seasonal behavior doesn't fit the pattern of flu-like seasonality. Especially, the beginning and ending. Some scientists have argued that kids going back to school after Summer is a major factor in the increase of flu-like incidence. But, then it's not explained why the ending of flu-like incidence is still during school periods. Outside the scope of this research, but something we could analyze in a 3rd study: what's the impact of lockdowns on the amplitude (peak) of flu-like incidence. If we specifically look at lockdown policies, we can observe that independent of the phase of Covid-19 (when it started in a country on the Northern Hemisphere), and independent of the degree of social distancing (Sweden virtually no lockdown, Italy/Spain/France hard lockdowns, Holland: intermediate lockdown), the endings of the covid-19 cycles all seem to be in sync. This doesn't imply that lockdowns are not effective: given seasonality, they are especially effective during flu-like season. And, TW and NZ show that a lockdown can also be instrumental in eliminating the covid-19 epidemic, which is probably easier for a relatively isolated island nation then for us. And, we would hypothesize that the lockdowns are helpful in "flattening the curve", independent of seasonality.

  • commented on June 5, 2021 by Neuromancer

    Umm….vitamin D3 levels anyone…….anyone??

    • Solar radiation (UV) as a factor is also covered by the research. It shows that it has an effect, though far smaller than pollen. The solar radiation effect can be explained both by melatonin and/or vitamin D3 -> immuno-activation. Alternatively, solar radiation is detrimental to the virus aerosol. It’s up to biomedical research to investigate if vitamine D3 is just a side-effect or an important factor.

  • commented on November 8, 2021 by Marcel

    I was wondering if the conclusion of your study (DOI: 10.1016/j.onehlt.2021.100277) still holds today. The study is limited to week 13, 2020 till week 5 2021. RIVM data shows that there was a major peak in COVID19 incidences per 100.000 in July 2021, week 22-26, 2021 (see graph at This peak was short but of visually similar proportion to the previous peaks in the fall and winter of 2020 but with a smaller tail. This peak, however, doesn’t seem to fit within the seasonal influenza pattern.

    • Hi Marcel, Yes, the conclusion still stands, and the seasonality of COVID-19 is widely accepted. Of course, also mobility plays a role. So, here we have compared an all-environmental with an environmental+mobility model (mobility explains of course the disco peak of Summer 2021).

      Although we chose a time period before vaccination, other studies show that the combination of environmental and mobility leads – like in our case – very good explanations of the reproduction number. In our case 87.5% of the variance of the Rt of COVID-19 is explained.

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