California Week1 H1N1 Hospitalizations Spike Higher
Recombinomics Commentary 23:45
January 13, 2014

The above graph from the California Week 1 flu report tracks pneumonia and influenza (P&I) cases as a percentage of hospital admissions in Kaiser Permanente Hospitals in northern California.  Like the CDC’s P&I death rate for the entire county, the levels oscillate throughout the year and pear at the height of flu season.  Like the CDC graph, the displayed data represents five flu seasons, and the above graph shows tha dramatic spike in recent weeks to a level higher than the past five seasons, even though the jump just began in the past few weeks.

The rate did get above the epidemic threshold last season when the dominant serotype was H3N2, which aggressively attacked the elderly, who usually account for >95% of flu deaths.  The dominance of H3N2 last season also led to a dramatic rise in the death rate for the entire country, which reached 9.8% of all deaths at its peak in early 2013.  This death rate, like the hospitalization rate in the above graph, was higher in the 2012/2013 season than 2009/2010, when pH1N1 emerged.

The death rate in 2009/2010 was blunted because pH1N1 spared the elderly because they have cross protection from seasonal H1N1 which circulated decades ago, when it was more closely related to swine H1N1.  As a result, flu deaths in the elderly were very low in 2009/2010, which is also true for the current season.

However, this season the current pH1N1 is causing a dramatic spike in the hospitalization percentage, even though the age groups hit hardest are young and middle aged adults.  This dramatic spike is also seen in Table 2 in the week 1 report from Dallas County in Texas, where cases increased several weeks ahead of California.  The Dallas report tracks hospital and ICU admissions for the current season, as well as 2012/2013, and like the above graph shows a dramatic uptick in cases which have already surpassed last season, even though the current pH1N1 attacks younger patients.

This dramatic increase in severe and fatal cases in young and middle aged adults is likely due to receptor binding domain changes which target human lung.  Changes such as D225G, Q226R, and L194I were noted in early case this season and additional examples were seen in new sequences released by the CDC from cases in late November or early December.  These changes were most easily seen in egg isolates, and all recently released egg isolates had these changes, highlighting the need for release of more sequences from eggisolates collected from more recent cases, such as those depicted in Texas and California.

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