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Influenza: Global Epidemic 2017 Versus The Flu Season Of 2019/20

The global influenza epidemic of late 2016 to early 2018 was a pandemic in all but name. For the United States, see The Perfect Storm Behind This Year's Nasty Flu Season, The Atlantic 14 Jan 2018; and Last year's flu broke records for deaths and illnesses new CDC numbers show, Washington Post, 26 Sep 2018. To see this event's impact on deaths in the United States in the winter of 2017/18, see this chart of mine, from Influenza context for covid deaths in selected countries, 2 Nov 2021.

In the 'Perfect Storm' article, the author (Sarah Zhang) says: "Fears of a bad flu season first began in the early fall, after public health officials noticed a worse-than-average flu season in the southern hemisphere". And "this year’s vaccine was only 10 percent effective against the problematic H3N2 strain in Australia". (This was the strain of influenza associated with the 1968 Hong Kong flu pandemic; refer to Fifty Years of Influenza A(H3N2) Following the Pandemic of 1968.) For Australia, see 2017 influenza outbreak, from Victoria's Department of Health, showing in a chart how much worse the 2017 event was than the 2009 Swine Flu pandemic. And see: What Australia's bad flu season means for Europe, North America, by Susan Scutti, CNN, 14 September 2017.

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(Note also, Flu Season 2017-2018: A Look at What Happened and What's to Come, by Laurie Saloman, 28 June 2018 [Contagion Live website]. This looks forward to a "universal vaccine"; indeed the January 2020 Netflix documentary series Pandemic, which featured that idea, was most likely inspired by the 2017/18 event.)

Here are my charts for Australia and New Zealand. Australia had an estimated 2,500 deaths over and above a normal flu season in 2017, and at least 5,000 more excess deaths that year than in its covid first covid year of 2020. That flu epidemic, in July 2017, represented New Zealand's stark mortality peak in the period since 2015. (In New Zealand, July 2019 and July 2021 also represented seasonal mortality highs; 2019, like 2017, also featured a nasty influenza.)

To understand the spread of the 2017 global influenza epidemic, it is instructive to look at the seasonal mortality chart for Greece. We see that 2017 was very bad there for excess mortality in January, June and August. And Italy was bad in January and August. Greece may have been the main transmission channel of this flu strain into Australia, given Australia's large Greek communities. And, at a time of considerable travel between Australia and southern Europe, this flu strain seems to have fed back into Europe around August 2017. In Italy in particular, this flu outbreak was strong in both December 2016 and December 2017. Italy will have been one of the routes through which this festering epidemic reached the United States.

This resurgence of H3N2 influenza seems to have begun in south and central Europe, with some countries there having a particularly bad 2016/17 flu season: in addition to Greece, there was Austria, Switzerland, France, Norway, Sweden, Finland, and Germany. In some of these countries – Germany, Denmark, Sweden, Finland (plus United Kingdom and Netherlands) – the 2017 flu returned with a vengeance in early 2018, peaking about two months after the United States peak. The lethal H3N2 strain circumnavigated the world, from December 2016 to March 2018.

It is looking like the post-2020 Covid19 mortality outcomes in the countries mentioned (and, of course, some other countries) reflects these countries' prior influenza experiences. After being badly (and twice) hit by the H3N2 global epidemic, Sweden went through two winters of minimal flu. Then it was initially hit particularly badly by Covid19, with death rates being concentrated among people aged over 85, the chief casualty cohort for influenza. Germany, which had a very bad flu experience in 2018 (and also a significant is not substantial experience in 2019) was hit much less by Europe's first wave of Covid19. Greece, the only European country to have a substantial influenza experience in January and February of 2020, was barely hit at all by covid's first European wave; whereas Italy, with a very light flu experience in early 2020, was very hard hit by Covid19.

In general, it looks like an unusually light influenza season in Europe in 2019/20 opened the door to Covid19; immunity to respiratory infections would have been lower than usual for March, and there were many older people alive in early March 2020 who would have already died had there been an average or above-average flu season.

A species – whether a micro species, or a macro species like humankind – moves quickly into an empty spaces. That's ecology. Once one species moves in, it tends to shut the door on would-be competitors aiming for that same space. The BBC reported "University of Glasgow scientists say it appears cold-causing rhinovirus trumps coronavirus" on 23 March (How the common cold can boot out Covid).

In this context we may note the New York Times warning of a 'twindemic' last northern winter (Fearing a ‘Twindemic,’ Health Experts Push Urgently for Flu Shots); a twindemic that didn't happen, presumably because covid trumped influenza. The Washington Post makes a similar prediction for the coming northern winter (Flu practically vanished last year. Now doctors are bracing for potential ‘twindemic’ of flu and covid-19 spikes); my sense is that, once again, covid will trump influenza, and that there will be no twindemic.

My sense, also, is that endemic (seasonal) covid – as it will be in the future – will be nothing like as bad as the next global influenza epidemic will be. The Americans are correct to place as much emphasis on getting seasonal 'flu jabs' as they do on getting regular 'covid jabs'. We in New Zealand are likely to be woefully unprepared for either covid or flu in the winter of 2022. (For priority New Zealanders to get biannual covid revaccinations in May 2022 before winter sets in, they will need to get their first revaccination before the end of this month.)

The global influenza epidemic of 2016 to 2018 was a particularly nasty assailant across the economically developed world of temperate latitudes. New Zealand's health officials should not dismiss, as relatively trivial, the possibility of another similar flu event. (The danger sign will be another 'good year' like 2016, in which mortality was unusually low.) 2017 may have been New Zealand's worst epidemic experience of a respiratory virus since 1918. Influenza vaccines are partially effective, sufficiently effective that they should be made available in 2022 to an extent similar to their availability in the United States this year.

Partially effective vaccines are not the only ways to get a degree of protection from a respiratory viral outbreak. It looks like infection from one respiratory virus may give some protection against another. It's for that reason that I do not think we will face a major influenza outbreak next winter, or even the winter after that. I will still be taking my annual influenza vaccines, though.

(And I will regard myself unvaccinated from Covid19 if I cannot get biannual coronavirus immunisations when I am due; I guess I should prepare myself to self-isolate from March 2022, just in case the government still has not rolled out covid revaccinations by then.)

Conclusion

The 2016-2018 global influenza epidemic was a beast. Not only did it trigger many deaths, it also made many more people so sick that they felt like they would die. We should not play down our awareness of our seasonal flu events, some of which will be particularly severe. We should not overreact to them either. Even the 1918 flu ‘event’ – worse than the Covid19 pandemic – in itself had little impact on the subsequent unfolding of history. By contrast, our nations' government-led responses in 2020 – through dramatic restrictions on mobility, the unprecedented peacetime reach of government power in liberal democracies, and the polarisation of political belief systems taken to new post-war levels – will have much more impact on future history than the virus on its own could ever have had.

PS

TVNZ's Seven Sharp programme yesterday evening featured an interview with Professor Graham le Gros, in which this issue of lost immunity to influenza, and the possibility of a winter twindemic in 2022, were the main themes.

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Keith Rankin (keith at rankin dot nz), trained as an economic historian, is a retired lecturer in Economics and Statistics. He lives in Auckland, New Zealand.

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