Epidemic Waves
Visualizing worldwide influenza and SARS-CoV-2 activity over time
August 15, 2020: Major updates to article and animations
January 23, 2021: All SARS-CoV-2 data for 2020 added in the world timeline
November 21, 2021: All SARS-CoV-2 data until October 31 added, animated maps updated and improved, Mexico state level influenza and Covid data added. Covid year-over-year comparison animated maps added.
Influenza viruses travel across the globe in a wave. In much of the northern hemisphere influenza activity picks up in late November, early December, peaks around February, and then starts to go down, only to emerge in much of the Southern hemisphere around May before reaching its peak there around July or August, then decreasing and reemerging in the Northern hemisphere. In the tropics in both hemispheres the situation is different and more complex, with influenza activity in many areas roughly tracking the rainy season.
Remarkably perhaps, there is no scientific consensus on exactly why influenza follows this pattern, why it peaks in the winter months and in the rainy season. It could have to do with the climate itself: The virus may survive better in colder and drier weather, although the latter wouldn’t explain why it peaks in the rainy season as well. Or it may be due to the lifestyle changes colder and wetter weather induces: People spend more time indoors, which may cause vitamin D deficiency, making people more susceptible to disease. Or more time indoors means people are in closer contact with each other, increasing the risk of transmission. These are just some of the possible explanations that have been proposed.
What about SARS-CoV-2 and the disease it causes, Covid-19? Like influenza, Covid-19 is a respiratory illness caused by a virus. Will it follow a similar seasonal pattern as influenza viruses do?
The problem is that to the extent that we are not sure what causes influenza’s seasonality or know just to what extent SARS-CoV-2 is similar to influenza in relevant respects, we do not have a good theoretical basis for predicting whether it will follow a similar pattern.
But we can observe what has happened so far. And that is what we do here. Starting in December 2019, for each month we show the typical influenza activity for that month as well as the number of new fatal SARS-CoV-2 infections that month.
Influenza and SARS-CoV-2 World Map and Timeline
For a more detailed look at these waves of influenza and SARS-CoV-2, you can go to the animated timeline.
Some important notes on how to understand the map and timeline:
The country colors — white and three different shades of blue — represent the different levels of historical influenza activity, as explained below. The darker the color the more common significant influenza activity is in that country in that month. Countries for which no influenza activity data was available remain white throughout the animation.
The bars represent SARS-CoV-activity. Height and color — also white and three different shades of blue — both represent the level of SARS-CoV-2 activity. The higher the bar and the darker the blue the more intense that activity.
The video directly below shows how you can use and control the animated timeline. In short:
- You can zoom pause, play and change the speed of the animation.
- You can click on countries and the bars for more information.
- And you can rotate and tilt the map by clicking ‘ctrl’ and moving your mouse.
How did we measure influenza and SARS-CoV-2 activity, respectively?
Let’s start with influenza.
Measuring Global Influenza Activity
Measuring influenza activity is an imperfect science, even more so when measurement spans the globe. Available data are imperfect and extrapolating from the data is fraught with difficulties. Our goal here was to get a rough sense or estimate of how influenza typically travels across the globe and then to see if SARS-CoV-2 follows a similar pattern. While influenza viruses are always present to some degree there are clear differences in the level of activity over the course of a year. And it is these seasonal differences in the level of activity that we are after.
That is why we chose ‘above average influenza activity’ as our metric. Specifically, we used data from the World Health Organization’s Global Influenza Surveillance and Response System’s FluNet to:
- look at the past three years (2017–2019) and calculate for each country the average level of influenza activity for a year
- then check for each of the 12 months of the year how many times influenza activity in that month was above the average for that year
- and finally, use this as the basis to give that month a score on a scale from 0–3
For example, if in France in the month of January there was above average activity in all three years, it receives a score of 3 (1 for each year). If there was above average activity in January in only one of those three years, it receives a score of 1. And when in none of the three years there was above average influenza activity that month, it receives a score of 0.
It is important to note that this scale of 0–3 does not express the average intensity of influenza activity in a month. Instead, it expresses how many times in the past 3 years influenza activity was above the annual average. If we had taken the average intensity for a month over that three year period, then if the intensity was extremely high in one year but below average in the other two years, it could have still come out as a high intensity month. But such a number would tell us less about what we are after: how influenza viruses typically travel across the globe, how common it is for there to be significant influenza activity in a given month.
Update November 2021: We changed the scale from 0–3 to 1–4. So what used to be 0 is now 1, what used to be 1 is now 2 etc.
We intentionally did not include data on influenza activity during the SARS-CoV-2 pandemic itself, both because the influenza data is not always available yet and because the SARS-CoV-2 pandemic may have skewed the data: Many healthcare systems were overburdened or had reduced access in anticipation of a surge of SARS-CoV-2 patients. A likely result has been that a meaningfully large number of people with influenza did not access healthcare services in the past few months and hence would now not be included in influenza statistics. It is also quite likely that at least some vulnerable patients with SARS-CoV-2 would have instead been hospitalized or even died from an influenza virus had SARS-CoV-2 not been around this year. So to get a sense of the typical wave of influenza activity it would have been unwise to include data from what may very well have been an outlier year, at least in terms of the data that is or will be available. (update February 11, 2021: Astonishingly, in 2020 influenza seems to have virtually disappeared worldwide)
Given the difficulties, caveats and choices mentioned above, the numbers we have arrived at should be treated with caution. We welcome criticism, comments and suggestions on how to improve our method of quantitatively measuring and visually expressing the typical wave of influenza activity across the globe. You can contact us at this address.
The data that is the basis for our scoring of global influenza activity is available here (zip). Specifically, we use the data in the ‘ALL_INF’ column as the measurement of influenza activity.
Influenza at the State or Provincial Level
The animated timeline above shows how influenza activity travels across the globe. It shows monthly changes in influenza activity per country, and because it does this for all the countries in the world, it shows the spatial and temporal pattern of influenza activity across the globe.
But the level of detail is limited. Some large countries have complex regional patterns of influenza activity. For example, as influenza activity peaks in the northern part of India in February and then starts coming down in March, it starts increasing in the southern and then central parts of the country to peak there in July and August. Such level of detail is lost by using only national and not local data.
That is why for some large countries we have also investigated and visualized influenza activity on a state or provincial level.
We did this for
video:
video:
video: see above
video:
For a world map of influenza activity that also includes this more detailed view of these four countries, go here (the map will load automatically when you click the link).
Please note that for now these more detailed maps at the state or provincial level only cover influenza activity. SARS-CoV-2 data at that level will be added in the future.
Also note that the data sources and methods we used to estimate influenza activity levels on the state or provincial level are different from those we used for country-level influenza data, and from each other. We list our sources and describe our methods for these local level overviews here.
Measuring SARS-CoV-2 Activity
Now that we have explained how we arrived at our overview of influenza activity across the globe, let’s return to SARS-CoV-2.
We measure SARS-CoV-2 activity by taking the number of fatal infections per million that occurred in a month. That is, we look at the number of SARS-CoV-2 infections that resulted in Covid-19 deaths.
Why use deaths and not cases as a proxy for SARS-CoV-2 activity? Data for cases depend to a large extent on how much testing is being done. With big increases in testing there can be big increases in the recorded number of cases without there being corresponding increases in the number of actual infections. It’s just that many infections that would have gone undetected in the absence of widespread testing are now recorded and included in the official statistics.
To be sure, that doesn’t mean there are no issues with using deaths data as a reliable proxy for SARS-CoV-2 activity. For example, there are measurement problems: Not all countries or regions use the same criteria to determine which deaths count as SARS-CoV-2 deaths. Some countries count everybody who tests positive for Covid and subsequently dies as a Covid death, no matter whether Covid was the primary cause or even meaningfully contributed to the death. Other countries use much stricter criteria. That makes it difficult to compare countries when it comes to the absolute numbers of Covid deaths, but it does allow for meaningful comparisons between countries when it comes to temporal patterns (unless during the year a country makes changes in the way it counts Covid deaths).
In addition, different countries have different demographics. Given that for example older people and obese people are more likely to die from Covid, and given that some countries have more old and/or obese people than other countries do, differences in death rates between those countries need not signify corresponding differences in virus activity. But here too temporal patterns between countries can still be meaningfully compared.
A different kind of methodological problem occurs as the result of the measures taken by societies to combat the virus and the damage it causes: Decreases in the number of deaths may be the result of those measures rather than of subsiding virus activity. For example, if vulnerable groups are better protected now, deaths may go down even though virus activity may be very similar.
Moreover, to the extent that treatment has been improving over time, a decrease in the number of daily deaths does not necessarily mean a decrease in SARS-CoV-2 activity.
Despite these caveats death rates still seem like a more reliable indicator of temporal patterns in the level of SARS-CoV-2 activity than cases are.
But deaths obviously lag behind infections. So if we use deaths as a proxy for virus activity, we have to account for this lag. How did we do this? Because the time between infection and death is typically roughly three weeks, we took the number of deaths from the period between the 22nd of that month and the 21st of the next month, to estimate the number of infections that occurred in that month and that ultimately turned out to be fatal.
For example, for the month of January the number of fatal infections that occurred that month is calculated by taking the number of Covid deaths that occurred between January 22 and February 21.
We used Covid deaths data from Our World in Data as the source for these calculations. For state/provincial level data we used Google’s Covid-19 Open Data. The results are available here as an Excel file.
Limitations
SARS-CoV-2 has only been known to the world since late 2019. And while we are learning more and more about the virus and how it is typically transmitted, and while we are finding that the virus does seem to spread more easily in settings that are more common during flu season, when it comes to the empirical question how the virus travels across the globe we have meaningful data for only the past year or so.
Therefore, it is quite possible that the correlation so far between influenza and SARS-CoV-2 activity may not be due to any meaningful and persistent similarities between the two types of viruses and the ways in which they typically interact with the changing environments in which they occur.
For example, we cannot exclude the possibility that the fact that SARS-CoV-2 emerged in China at the start of flu season rather than in some other time (and/or place) of the year was relatively meaningless and coincidental. Maybe it could have just as well emerged in May or July instead and started its spread across the globe several months earlier and/or in different ways than it has now. And if that had happened, there may have been less overlap with influenza activity patterns.
So we caution against drawing simple causal conclusions merely on the basis of the data presented here.
With more time will come more data, which is why we will continue to update this article and the animations as the pandemic unfolds.
Team: Koen Swinkels, Conrad van Maanen, Kasper de Rijke, Maurice de Hond
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