Predicting The Next Influenza Season

Last month, in August 2019, we have our first influenza case in our hospital. The patient recently returned from an international gathering so it is likely that he acquired the flu from a Southern Hemisphere resident attending the same gathering. The Southern Hemisphere has its influenza season at the opposite time of year as the Northern Hemisphere. A 2019 article in the journal Scientific Reports indicates that the epidemiology of influenza in Australia in any given season predicts the subsequent epidemiology of influenza in the Northern Hemisphere. So, what does the most recent Australian influenza epidemiology indicate?

The Australian Department of Health maintains a robust website with up to date influenza epidemiology information. The 2019 Australian data indicates that influenza started early this season as is depicted in the red line in the graph below.

Normally, influenza starts showing up in late May in Australia and peaks about the first of September. Australia started the onset of the 2019 influenza season in February, about 3 months earlier than usual, and it peaked about the first of July, about 2 months earlier than usual. So, let’s see how the Australian influenza data compares to information from the U.S. Centers for Disease Control influenza website:

In the above graph, we see the last 5 influenza seasons plus the 2009-10 season (the H1N1 epidemic) and the 2011-12 season. In 2009, the U.S. was caught off guard with the early arrival of H1N1 influenza that started in April and peaked in October. That year, 61,000 Americans got influenza and 12,500 Americans died of it (especially young persons). The 2017-18 influenza season (turquoise line in the graph) had the highest percentage of visits for influenza-like illness in the past 5 years. Comparing that curve to the 2017 Australian influenza season (brown line in the previous graph), there is remarkable similarity.

If the upcoming influenza season in the United States resembles the current influenza season in Australia, then it will start earlier than usual, perhaps in October rather than the normal December onset. The good news is that the most recent influenza season in Australia was not terribly severe with fewer hospitalizations and deaths than normal so hopefully this portends good news for U.S. hospitals and ICUs that may see fewer admissions and deaths from influenza.

What about the specific strains of influenza? Last year in the United States, influenza A H1N1 dominated early in the season in December (brown bars in the graph below). Influenza A H3N2 became the dominant strain by early March (red bars in the graph below). Influenza B was uncommon throughout the 2018-19 season until February and then it increased in prevalence when influenza A was decreasing (green bars in the graph below).

 

 

The graph below shows the various strains seen in Australia in the summer of 2019. Once again, influenza A peaked early in the season (red bars in the graph below) whereas influenza B peaked 4 months later (green bars in the graph below). Overall, influenza B accounted for a higher than usual percentage of the overall influenza cases in Australia. Influenza H1N1 all but disappeared shortly after the Australian flu season started and H3N2 was the dominant strain (although most of the Australian influenza A cases went unsubtyped). If the Australian experience predicts the upcoming flu season in the United States, then we should expect to see mostly influenza A H3N2 and also see a higher than normal number of influenza B this winter.

The 2019-20 U.S. standard strength influenza vaccine is quadrivalent and will cover the following 4 strains:

  1. Influenza A/Brisbane/02/2018 (H1N1)pdm09-like virus
  2. Influenza A/Kansas/14/2017 (H3N2)-like virus
  3. Influenza B/Victoria virus
  4. Influenza B/Yamagata virus

The vaccine is available in a standard strength that is indicate for persons over age 6 months and is made by several manufacturer, sold under brand names including Alfuria, Fluzone, Fluarix, and FluLaval. A high strength (Fluzone High Dose) contains about 4 times the amount of antigen and is indicated for persons over age 65. Unlike the quadrivalent standard strength influenza vaccine, the Fluzone High Dose vaccine is trivalent and does not cover the influenza B/Yamagata virus. This could have immunity implications if the U.S. sees an unusually high number of influenza B cases, as was seen in Australia this summer. Also, the Fluzone High Dose vaccine has a higher incidence of side effects such as injection site pain, fever, and myalgia. So the decision of which vaccine to use in persons age > 65 is not clearcut – the CDC does not recommend one vaccine over the other in people over age 65.

The good news for the upcoming influenza season is that we now have a new drug to treat influenza, baloxavir (trade name Xofluza®) that is given as a single one-time dose and will cost about $150. Tamiflu® is now available as generic oseltamivir at a cost of about $50 for a 5-day course.

So, what can physicians in the United States take away from this season’s Australian influenza season? Vaccinate your patients and do it now!

September 28, 2019