The influenza hospitalization rate for children and young adults has surpassed the rate documented during the second wave of the 2009 H1N1 pandemic.
The latest data from the US Centers for Disease Control and Prevention (CDC) indicate that there have been at least 32 million cases of influenza in the 2019-20 US flu season.
Have we passed the peak of flu season?
It may be too early to tell, however, the rate of outpatient visits for influenza-like-illness decreased nationwide for the second week in a row. Additionally, the percentage of specimens testing positive for both influenza A and B viruses decreased last week.
While a decline in influenza-like-illness could be a sign the season is slowing down, key indicators that track flu remain high overall. Currently, 45 jurisdictions are experiencing high influenza-like-illness activity and 50 jurisdictions are reporting regional or widespread influenza activity. So far, there have been 310,000 flu-related hospitalizations and 18,000 associated deaths this season.
The rate of hospitalization remains similar to what has been seen at this point in the flu season in recent years for adults and older adults. Overall, the cumulative hospitalization rate for the season stands at 52.7 per 100,000. However, hospitalization rates among children and young adults are higher than usual.
According to the CDC, for the younger populations, hospitalization rates have reached the highest on record since influenza reporting began in 2004-5. The hospitalization rate for children and young adults has surpassed the rate documented during the second wave of the 2009 H1N1 pandemic.
The percentage of deaths related to pneumonia and influenza is currently 6.9%, which is below the epidemic threshold of 7.3%. While mortality for this season is considered low, there have been 125 influenza-associated deaths in children thus far this season. This figure is also higher than every season since reporting began, with the exception of the 2009 pandemic.
FluView data also indicates that the percentage of specimens positive for influenza at clinical laboratories decreased from 29.7% last week to 26.4% this week. At this point, influenza B/Victoria and A(H1N1)pdm09 viruses are approximately equal for the season overall, and continued increases in influenza A(H1N1)pdm09 viruses have been seen in recent weeks.
The predominant influenza virus continues to vary by age group. Nationally, influenza B viruses are the most common among children and young adults under 25 years. On the other hand, influenza A viruses are the most commonly reported among individuals 25 years and older.
The CDC continues to recommend vaccination with the seasonal flu shot. The most recent data indicate that 174.1 million doses of the flu vaccine have been distributed.
Two weeks ago, the CDC’s Morbidity and Mortality Weekly Report featured interim estimates of the effectiveness of 2019-20 flu shot. The report estimates that so far this season the vaccine is 45% effective overall and is 55% effective in children.
Although the authors of the report acknowledged that more effective vaccines are needed, they also noted that the currently available vaccines are providing substantial health benefits.
“Most years the mortality rate from influenza is a U-shaped curve where the mortality is the highest in the very young and in elderly patients and that's due to immune response, but also comorbid diseases, particularly in the elderly. Comorbidities are really what people put people at the highest risk of complications from influenza. Asthma, COPD, other pulmonary conditions, even heart conditions, and the immunocompromised population, all of these patients are at a significantly higher risk of complications from influenza than the general population,” Jason Gallagher, PharmD, Contagion®’s editor-in-chief explained.
Contagion® will continue to monitor the US flu season and provide updates as they become available.