Current Status of the 2023-24 Influenza Season in the United States
Despite Australia experiencing a severe flu season, the United States has not yet seen a corresponding increase in influenza activity, which is unusual given the historical pattern of the Southern Hemisphere's flu season predicting trends in the Northern Hemisphere.
Typical Influenza Season Patterns
- Influenza seasons in the United States typically begin to increase in the fall and can peak at variable times, though most often in winter months 1.
- In 74% of influenza seasons from 1982-83 through 2015-16, peak influenza activity did not occur until January or later, and in 59% of seasons, the peak was in February or later 1.
- Historically, the United States' flu patterns often mirror Australia's previous season, which serves as a predictive indicator for Northern Hemisphere countries 2.
Factors That May Explain the Current Situation
- The timing of influenza activity varies considerably from season to season, with some seasons not showing significant activity until February, March, or later 1.
- The duration and severity of influenza seasons are unpredictable, and more than one outbreak might occur in a given community in a single year 1.
- The COVID-19 pandemic significantly disrupted typical influenza patterns globally, with dramatically reduced influenza activity observed during 2020-2021 due to widespread implementation of COVID-19 mitigation measures 2.
Potential Explanations for Delayed Onset
- Continued implementation of some COVID-19 mitigation measures (e.g., face masks, social distancing, increased hand hygiene) may be contributing to reduced influenza transmission 2.
- Vaccination efforts that began early in the season may be providing community protection, though effectiveness varies by season 1.
- Seasonal variability in virus circulation patterns is common, and the U.S. may simply be experiencing a delayed start to the season 1.
Implications and Recommendations
- Vaccination efforts should continue throughout the season since influenza activity might not occur in certain communities until February, March, or later 1.
- Even if influenza activity has already begun in a community, vaccination administered in December or later is still likely to be beneficial in most influenza seasons 1.
- Healthcare providers should remain vigilant for influenza infections and continue testing for seasonal influenza viruses despite the current low activity 3.
- High-risk populations should be prioritized for vaccination, including children aged 6-59 months, adults ≥50 years, those with chronic medical conditions, pregnant women, and residents of long-term care facilities 1.
Monitoring and Preparedness
- Despite current low activity levels, healthcare systems should maintain preparedness for potential influenza outbreaks later in the season 1.
- Continued surveillance is essential as influenza patterns can change rapidly, and multiple waves of activity can occur within a single season 1.
- The unpredictable nature of influenza circulation means that a significant increase in cases could still occur later in the 2023-24 season 1.
While the current flu season in the U.S. has not yet shown the expected increase in activity despite Australia's severe season, historical patterns suggest that significant influenza circulation could still emerge in the coming months. Continued vaccination efforts and vigilance remain essential throughout the influenza season.