In 2020, after significant interseasonal circulation of influenza between January and March, Australia saw the ‘arrival’ of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). The associated control mechanisms (mask wearing, travel restrictions, physical distancing, workplace and school closures) led to influenza numbers from April to September being less than 5% of the previous five-year average, and less than 0.5% of the number for the previous year’s number of almost 280,000.1 This disappearance of influenza cases was aided by a high influenza vaccination coverage – the distribution of 18 million vaccine doses in 2020 was a new record. Surveillance data (albeit inaccurate because many doses are not added to the Australian Immunisation Register) showed a 6% increase in vaccination coverage in older people and a record 44% coverage of children between 6 months and 5 years of age.5
Vaccination campaigns targeting avoidance of the ‘twindemic’ of concomitant coronavirus disease 2019 (COVID-19) and influenza infection were important in increasing vaccine uptake and decreasing influenza numbers. More important – almost certainly – were the public health measures used to address COVID-19, including a six-month lockdown of the city of Melbourne.
It was not just influenza numbers that declined during the winter of 2019 as a result of public health messaging. Many other respiratory infections did not circulate at their usual intensity. In Western Australia, notifications of respiratory syncytial virus (RSV) in children dropped by 98% during winter, despite schools being open.6
In much of the northern hemisphere in the winter just gone, SARS-CoV-2 was rampaging, aided by a very poor uptake of many public health measures. Helpful measures such as mask wearing became political issues and there was a widespread lack of compliance. Despite this, influenza did not arrive in either Europe or North America. The US Centres for Disease Control and Prevention described the northern winter influenza season case numbers as unusually low, including in children, with one recorded death last winter.7
Researchers from Public Health England looking at data for coinfection suggest a possible pathogenic competition between SARS-CoV-2 and influenza viruses may have contributed to low influenza numbers.8 Influenza is not a colonising virus, so competition is plausible.
Additionally, the marked reduction in international travel almost certainly played a part in the demise of a virus that travels the world from winter to winter.
What can we expect for COVID-19 in the upcoming winter?
SARS-CoV-2 will continue to be a risk to Australia through overseas arrivals. This RNA virus will continue to make mistakes copying itself, leading to variants that will outcompete older strains based on better infectivity. As with recent ‘variants of concern’, some of these new strains will produce more severe disease and some will have spike proteins that are sufficiently distinct to no longer be neutralised by antibodies that are produced by receipt of our vaccines.
The present Australian Government advice is that Australia’s external borders will officially stay closed until the end of June 2021. Whether a high vaccination coverage will lead to an opening of our borders then is unclear. If it does, SARS-CoV-2 will return to the country. This coronavirus is not amenable to eradication in the long term after borders re-open. Eventually we will need to learn to live with it, as we do with influenza each winter.
This winter, although the number of COVID-19 cases will continue to be high in many parts of the world, they will be minimal in a closed-border Australia.