By: Dr. Geoffrey Modest
The CDC sent out a health alert based on initial evaluations of influenza types in the US, noting a significant mismatch with those covered by the vaccine (see here). They note:
–Influenza A (H3N2) is so far the most frequently reported influenza virus found (though influenza is still pretty uncommon, they have found influenza A (H3N2) in almost all states). So far, 91% of 1228 positive samples have been influenza A.
–In the past, influenza A (H3N2) has been associated with higher overall and age-specific hospitalization rates and mortality (esp in the old, young and those with chronic medical conditions)
–Only 48% of the influenza A (H3N2) analyzed so far are antigenically similar to the influenza A (H3N2) vaccine component, with 52% showing antigenic drift (only 85 samples tested). Of note, the vaccine-types chosen each year are set the prior February, too late to change the vaccine now.
–This type of antigenic drift is associated with decreased vaccine effectiveness. Though there may be some cross-protection. also, no evident antigenic drift for influenza A (H1N1) or influenza B
–So, this finding still reinforces the importance of the vaccine even in its reduced-effectiveness state, given the annual influenza mortality of 3,000-49,000 people in the US over the past 30 years, but especially the importance of the neuraminidase inhibitors — oseltamivir (Tamiflu) and zanamivir (Relenza) — both for treating the flu (shorter duration of symptoms, reduced risk of complications, reduced risk of death in hospitalized patients), and for prevention in high risk situations. And these drugs should be given as soon as possible after symptoms begin, preferably within 48 hours. All viruses tested so far are susceptible to these drugs.
–I would add, from previous years, that when influenzal illnesses first begin in a community, it is useful to do lab test for flu (though don’t wait for results before treating with neuraminidase inhibitors). But when influenza is established in a community, the likelihood of someone with typical symptoms having influenza is so high that it is not useful or necessary to do lab tests (ie, the post-test probability doesn’t change much when the pre-test probability is high, based on symptoms and prevalence of of influenza in the community). just treat them.
I would add to the above that this blog post of 2 Cochrane reviews of neuraminidase inhibitors finding that
–There was significant drug company malfeasance in releasing the raw data
–Osteltamivir has better evidence of efficacy than zanamivir
–The benefits are not very dramatic (decreased symptoms of 16.7 hours, with no difference in hospital admissions, pneumonia or any serious complications)
–There were significant adverse events (nausea, vomiting, increased QTc, psych effects, headaches)
Reviewing the studies themselves show a remarkable diversity of outcomes with neuraminidase inhibitors, with some showing significant reductions in pneumonia and mortality and others not. so, it is really hard for us mere mortals to be sure…. My practice has been to use only osteltamivir (not zanamivir), only when studies show minimal resistance (as appears to be the case so far this year), and especially in more symptomatic people (high fever) or significant medical comorbidities.