H3N2

Yesterday many US newspapers carried front-page stories on the severity of influenza so far this season. The New York Times story began with “It is not your imagination — more people you know are sick this winter, even people who have had flu shots.” Is this really a bad flu season?

Before we answer that question, I would like to complain about what the Times wrote: ‘more people you know are sick this winter, even people who have had flu shots”.  A similar sentiment appeared in a recent Forbes column “Influenza-like-illness is sweeping the country with the Centers for Disease Control & Prevention reporting that most areas of the country experiencing high rates. I should know, my family is in the midst of it despite having been vaccinated.”

Remember that having a respiratory illness does not mean that you have influenza – it could be caused by a number of other viruses which of course would not be blocked by influenza vaccine. Furthermore, the influenza vaccine is not 100% effective – it’s about 60-70% effective in individuals younger than 65 years. That’s not great, but it is better than having no vaccine. The point that I want to make is that it is not useful for anyone to relay anecdotal information about immunization and infection unless you know for sure that you had influenza virus. It only further discourages widespread immunization, which is already isn’t where it should be (~40%).

To answer my question  – is this a bad flu season? – I looked at data from the Centers for Disease Control and Prevention, which receives thousands of respiratory specimens from laboratories throughout the US and determines if they contain influenza virus, and if so, which subtype. Here are the results through week 1 of January 2013:

According to these data, there was a peak of influenza activity in week 51 (December 2012). This is early compared with recent influenza seasons. In the 2011-12 season, influenza activity peaked in week 11 (March) of 2012:

During the 2010-11 season, the peak of influenza activity was week 8 (February) of 2011:

During the 2009-10 season, the peak of influenza activity was quite early, in week 42 (October) 2009:

The number of diagnosed infections each year is also indicative of the extent of the influenza season. So far in the 2012-12 season there have been 28,747 influenza positive specimens. Numbers in the previous years: 157,449 in the 2009-10 season, and 55,403 in the 2010-11 season (I was not able to locate totals for 2011-12).

Pneumonia and influenza mortality has so far not substantially exceeded the epidemic threshold as it has in previous years:

Pediatric deaths from influenza are on track to exceed last year’s total but not the previous two years:

The percentage of outpatient visits for influenza-like illness (based on symptoms, not virus isolation) is following a pattern that resembles recent moderately severe influenza seasons:

The New York City Department of Health and Mental Hygiene also monitors influenza and produces weekly summaries during the season. One metric they report is the percentage of visits for outpatient influenza-like illness. The curve resembles that for 2010-11:

New York City also identifies what type of respiratory virus is associated with influenza-like illness, including influenza viruses, adenovirus, respiratory syncytial virus, parainfluenza virus, and metapneumovirus. For week 1 of 2013 all isolates (n=45) were either influenza A/H3N2 or B, although analysis of about a third of them is pending. In previous weeks a mix of other viruses were found in addition to influenza.

Note also the prevalence of influenza H3N2 this season, which has largely displaced the 2009 pandemic H1N1 strain. The H3N2 subtype is generally associated with more severe influenza seasons.

In summary: the data so far suggest that influenza activity is peaking early than in the past two years, but not at an unprecedented time. Numbers of infections, pneumonia, and mortality, are not off the charts. I would agree with Jean Weinberg, a city health department spokeswoman, who said “This is not a season that is out of the ordinary, though H3 seasons tend to be worse than H1 seasons”.