Across the globe, measures that aimed to curb the transmission of COVID-19, including travel restrictions, wearing masks, and other strategies have also helped to limit the spread of influenza and other infectious diseases.

Consequently, in 2020–2021, the US and many countries in Europe observed their lowest influenza levels in recent years. However, with the continued circulation of COVID-19 and easing pandemic restrictions, many countries face the risk of rising case numbers for both COVID-19 and influenza this winter – a phenomenon coined the “Twindemic”. In the US, the Centers for Disease Control and Prevention (CDC) recently issued updated flu guidelines to mitigate the effects of the so-called Twindemic. GlobalData epidemiologists expect that the revised guidelines paired with the introduction of the COVID-19 vaccine in 2021 will be critical to mitigate the surge in influenza and influenza-like illness (ILI) in the 2021–2022 season.

The primary update to the CDC immunization guidelines requires all administered vaccines to be quadrivalent, providing protection against the four major viral strains of influenza: A, B, C, and D. This will ensure a more comprehensive level of protection for vaccine recipients compared to in previous years. An individual’s flu immunity is determined by several factors, such as previous exposure to influenza, vaccination history, and how closely this year’s vaccine matches commonly circulating strains. Flu seasons vary in length and severity depending on the predominant viral strain and the level of immunity of the population. Many countries routinely immunize infants, individuals with chronic health conditions, people over the age of 50, pregnant women, and healthcare workers. Vaccination coverage in the US is more expansive, recommended for anyone over the age of six months without underlying comorbidities. In the US’s 2018–2019 flu season, more than 75% of deaths were among individuals over the age of 65, as is the case with COVID-19.

According to GlobalData’s forecast for 2021, the US is expected to observe in excess of 1,400,000 diagnosed incident cases of lab-confirmed seasonal influenza and 16,900,000 diagnosed incident cases of ILI. However, the low infection levels seen last year have likely led to low natural immunity in the population. Consequently, there will be more people susceptible to infection this year than in a typical flu season. Current vaccination coverage is estimated to be around 45.8% of the eligible population. Unless there is a sufficient increase in vaccination coverage to compensate for waning natural population immunity, GlobalData epidemiologists expect there will likely be a surge in influenza infections and ILI this winter. In the advent of vaccine misinformation and anti-vaccination campaigns, this level of coverage may prove to be particularly challenging to achieve.

As both pathogens are circulating during the same season, people may be exposed to both influenza and COVID-19 simultaneously. COVID-19 and influenza co-infected patients are reported to have higher mortality rates and disease severity compared to patients with isolated infections. Therefore, the importance of a successful influenza vaccination campaign cannot be overemphasized in order to prevent the spread of influenza and COVID-19. Reduced severity of influenza illness from vaccination should lead to fewer outpatient visits and hospitalizations, easing the strain on the US health care system. Finally, vaccination will reduce influenza symptoms that could be confused with COVID-19, allowing for a faster diagnosis and treatment of both infections.