Where Is Influenza?

This is an edited version of an article I wrote recently for Pandemics Data and Analytics. I have added some personal opinion in the first two paragraphs. It is yet to be edited/published by Panda.

Dynamics between viruses and human populations are complex and not well understood.  Each year across the globe between 3 million and 5 million severe cases of Influenza illness occur, leading to between 300,000 and 650,000 deaths which can be from respiratory, cardiovascular and neurological complications. An estimated 500 New Zealanders die from Influenza each year. In Australia, Influenza and Pneumonia are reported as the ninth leading cause of death in 2019, with 4,124 deaths recorded. Severity of seasons, affected by multiple factors, impacts these numbers significantly from one year to the next. Most of these deaths occur in the over 65 year age group although children under 5 years old are also considered high risk.

This context is largely absent from mainstream media when reporting on COVID-19 case counts and death rates. Context would alleviate a lot of the fear that has been generated. I overheard a New Zealand woman speaking a week ago with high anxiety about her perception that “if COVID gets into New Zealand, it is going to cull the population”. This is completely incorrect, even if the New Zealand population remain vulnerable to this virus, and shows that her information source (probably mainstream media) has not offered any data about COVID-19 infection fatality rates or age graduation of disease severity. And what if New Zealand already have immunity as is being seen in large parts of the world who have no capacity to lockdown as New Zealand have? The idea that lockdown is the only factor influencing disease dynamics, and the unlikely credit given to lockdown when it opposes evidence based public health recommendations is disconcerting. Especially when strict lockdowns in other locations have resulted in extremely negative consequences (eg Peru). Unquestioning incuriosity driven by fear and panic is causing enormous harm.

As many as 200 other respiratory viruses also cause illness, death and sometimes outbreaks.  People who are unwell with viral lung diseases usually present with very similar symptoms meaning that without a laboratory test, specific diagnosis is not possible.  Because treatments for viral lung disease are generally the same regardless of which virus is responsible, tests are often not taken and when they are, laboratory tests can only detect a narrow range of the most common viruses.

Tests for Influenza (“the flu”) became available in the 1930s and World Health Organisation began the Global Influenza Surveillance and Response System (GISRS) in the 1950s.  The GISRS continues today, receiving data from national surveillance systems to provide a global picture of the patterns of Influenza disease.   Because routine laboratory testing for Influenza is not commonplace, these systems use a range of strategies including syndromic surveillance which monitors symptoms, severity and patient risk factors such as age and underlying health conditions.  In cases when physicians choose to test individual patients for specific diagnosis, and in those services which act as sentinel testing sites for surveillance purposes, positive Influenza results are reported to the surveillance systems.

Influenza can change parts of its structure, known as mutation, which occurs slowly but constantly.  Mutations are monitored for the purpose of understanding circulating Influenza patterns and to help vaccine producers predict which strains to use when producing the next season’s Influenza vaccination.  Influenza patterns amongst populations change so that between seasons, different strains of Influenza A will take over from each other as the dominant virus, as though competing against each other constantly.

Whilst Influenza A has been responsible for all known Influenza pandemics, there is speculation that the so-called Russian Flu pandemic of 1889-1890 may in fact have been caused by the human coronavirus known as HCoV-OC43.  Today this virus is one of four circulating human coronaviruses (excluding SARS-CoV-2) which cause up to 30% of common colds.  Coronaviruses sometimes cause outbreaks in high-risk populations such as long term care facilities accommodating the frail and elderly.

This year the flu appears to have almost completely disappeared.

This disappearance is being attributed to people following COVID-19 mitigation measures such as social distancing and wearing masks. However, given the complex relationship between viruses and their human hosts, this seems unlikely.  A systematic review in May 2020 of 14 randomised control trials found that hand hygiene and masks had no effect on laboratory-confirmed Influenza.  Influenza has disappeared globally, including places where mitigation measures are not practiced and across locations with differing measures being implemented.  The limited impact of mitigation measures on COVID-19, which transmits in the same way as Influenza, strongly suggests that human behaviour is unlikely to be responsible for the disappearance of Influenza.

Influenza, SARS-CoV-2 and the hundreds of other respiratory viruses responsible for infectious respiratory illness share many similar symptoms.  Without a laboratory confirmation it is not possible to determine which virus is responsible for any individual case or outbreak.  It is possible that in previous seasons, in the absence of comprehensive testing, Influenza may have been attributed to other respiratory viruses.  Equally, during the COVID-19 pandemic, it is possible that SARS-CoV-2 is the assumed organism where testing is not performed, or the only confirmed organism in the absence of testing for other causes.

World Health Organisation state that the COVID-19 pandemic has influenced on a global basis, factors such as health seeking behaviours, capacity of sentinel sites to continue their usual work, and testing priorities and capacity.  In some places Influenza testing is reported to have increased.  However, laboratories have significantly escalated capacity in relation to PCR testing for SARS-CoV-2 during the pandemic.  For example, UK laboratories were initially able to perform less than 20,000 SARS-CoV-2 tests per day.  At the end of January 2021 this is reported to have increased to almost a million daily tests.  This may have impacted the usual Influenza testing and reporting capacity of laboratories.

The COVID-19 pandemic has altered the way that people access health care.  People with respiratory symptoms will now often seek a PCR test to exclude COVID-19 before attending health services for further care.  Influenza tests are most reliable when samples are taken within the first three days after symptoms begin.  It is very unlikely that all those who test negative for COVID-19 are followed up with a flu test, and of those who are, many may be tested beyond the reliable timeframe of less than 72 hours since symptom onset.

It is possible to be coinfected with different respiratory viruses at the same time and there have been confirmed cases of COVID-19 with Influenza and other viral lung infections.  Given the fluctuating patterns of winter respiratory viruses, there is also a possibility that upon entering the transmission chain, SARS-CoV-2 has supplanted Influenza as the dominant virus strain.

A new test is being rolled out in some locations which tests simultaneously for SARS-CoV-2 and Influenza.  This may be useful in solving the puzzle of Influenza’s disappearance.  Solving this mystery is important because populations are best protected when circulating microorganisms allow immunity to establish.  The absence of Influenza for a whole season may jeopardise levels of protective population immunity, threatening a serious Influenza season if it really has been absent, and returns next year.

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