What has happened to influenza (and everything else) during the COVID-19 pandemic?

By December 10 2020, there have been over 68 million cases of COVID-19 and over 1.5 million deaths worldwide. But there has been a dramatic reduction in influenza, a winter illness responsible for about 3000 deaths annually in Australia

As these are laboratory-confirmed cases the true number is likely to be much higher as not everyone seeks medical care and testing access in many countries is limited.  In Australia, there have been almost 28,000 cases and 908 deaths, with the bulk of these occurring in the second wave in Victoria in mid-2020.  Australia and New Zealand are lucky in their remoteness, so that the banning of inbound travel allowed the opportunity to successfully implement physical distancing and improved infection control practices.  Given there are no effective antiviral drugs and licensed vaccines, these public health responses are remarkably similar to those undertaken in Australia during the 1918-19 influenza pandemic.

An unexpected outcome of the COVID-19 pandemic in Australia has been the dramatic reduction in influenza, a winter illness responsible for about 3000 deaths annually.  On the 20th March 2020, Australia introduced an entry travel ban on foreign nationals, and over the next few days a number of states and territories also closed their internal borders.  Since then, all measures of influenza activity, such as presentations to general practice, admissions to hospitals and intensive care units, and laboratory testing have shown a similar dramatic reduction.

This decrease has been almost absolute, and one could say that influenza is genetically extinct in Australia for the first time.  A paper published in late November in Eurosurveillance (the journal of the European Centre for Disease Prevention and Control) showed that there were only 36 influenza attributable deaths between January and September 2020, compared to 812 in the comparable period in 2019.  There has been only one influenza confirmed admission to the fifteen national sentinel hospitals since April 2020 (1).

This has proved a blessing for public health and hospital management, as authorities were expecting the usual winter surge in influenza activity to coincide with COVID-19.  As part of the management of the first COVID-19 wave, the Australian population was strongly encouraged to have the influenza vaccine from April 2020.  However, although vaccination rates were high, vaccine effectiveness would never have cause such a drop in influenza.

Interestingly, the reporting of other respiratory viruses, respiratory syncytial virus (RSV) and rhinoviruses, has risen.  However, the oddity is that this rise has occurred much later than the normal winter peak.  RSV causes significant disease in very young children (and sometimes in adults) and usually occurs in early winter.  Rhinoviruses are responsible for the common cold, and their frequency is probably related to increased testing in people with otherwise minor symptoms to exclude COVID-19.  The fact that RSV and rhinoviruses have occurred so late in the year may imply that the physical distancing and infection control measures have relaxed, allowing the viruses to re-surface.  It may be that these viruses circulate in a cryptic fashion in the community, and so as restrictions have eased they have re-emerged.  In contrast, influenza is most likely imported into Australia each year by travellers from the northern hemisphere, and once this stopped influenza circulation ceased.  Viral genetic studies are underway to examine this hypothesis.

The improved physical distancing, infection control practices and restrictions of gatherings has meant that gastrointestinal viruses have also decreased.  Noroviruses are the most important of these viruses, responsible for outbreaks of foodborne disease in the community and in ‘closed’ environments such as aged care facilities, cruise ships and childcare centres.  Again, as restrictions have eased in Australia we have seen an increase in norovirus infections, particularly in childcare centres.  At least the closure of the cruise ship industry has stopped further outbreaks of norovirus and respiratory virus infections in this ‘closed’ environment.  The shadow of the Ruby Princess looms large…

Increased deaths from non-COVID-19 causes during COVID-19 waves has been reported.  In the USA, where the COVID-19 pandemic has been especially severe, deaths from diabetes, Alzheimer’s disease and dementia, hypertension, pneumonia, coronary artery disease and stroke have all been higher than normal.  Some of these deaths may be directly attributable to unrecognised COVID-19, or to the pandemic-induced disruption to hospitals and the healthcare system.

Whether this has occurred in Australia is under investigation, but the COVID-19 pandemic has been mild in comparison to much of the world.   Even during the usual Australian influenza season, there is an increase in pneumonia, cardiac and cerebrovascular disease, outcomes that that can be improved with better influenza vaccination of at-risk community members.  There is speculation that other infectious and non-infectious medical conditions, may rise as restrictions ease as routine doctor visits, cancer screening, dental work, cardiac and diabetes assessments all decreased during the COVID-19 lockdown.  It remains to be seen if this leads to increased disease rates over the next year or so.

It is likely that SARS-CoV-2 will circulate in the human population for the foreseeable future, even after introduction of a successful vaccine (smallpox in humans and rinderpest in animals remain the only viruses to be declared eliminated with vaccination by the World Health Organization).  The logistics of rolling out SARS-CoV-2 vaccination to 8 billion people worldwide, remembering that each person is likely to require two doses, are enormous and it is clearly going to take some time to achieve.  In Australia, ensuring vaccine ‘buy in’ by the population will be crucial to this effort, and will be important to ensure that other vaccine programs are not affected.

Many of the lessons learnt from the COVID-19 pandemic are not new – they are no different to those from the 1918-19 influenza pandemic and other outbreaks in the last century.  The use of ‘old fashioned’ physical distancing, infection control practices and intermittent lockdowns remain effective if intelligently deployed and explained to the community.  There are new lessons – better managing aged care, protecting our healthcare workers, ensuring bipartisan political solutions, maintaining local capacity in essential scientific and clinical equipment production, harnessing new technologies in science and data management, and coordinating research expertise, amongst others.  These lessons will be important to remember as the local and international economies open up and will apply not just to COVID-19 but to influenza and other important pathogens.

 

  1. Sullivan SG, Carlson S, Cheng AC, Chilver MBN, Dwyer DE, Irwin M, Kok J, Macartney K, MacLachlan J, Minney-Smith C, Smith D, Stocks N, Taylor J, Barr IG. Where has all the influenza gone? The impact of COVID-19 on the circulation of influenza and other respiratory viruses, Australia, March to September 2020. Eurosurveillance 2020; 25(47):pii=2001847.
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Dominic Dwyer is a medical virologist and infectious diseases physician. He is currently the Director of Public Health Pathology for NSW Health Pathology, and is Director of the Institute of Clinical Pathology and Medical Research at Westmead Hospital. He is a Clinical Professor at Sydney University, and is part of the University’s Marie Bashir Institute for Emerging Infectious Diseases and Biosecurity.

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