This year's influenza season, while mild so far, comes with a few of Mother Nature's curveballs that will impact our approach to prevention and treatment.
Normally, peak influenza activity hits by mid-January, and as of mid-January this year, the Centers for Disease Control and Prevention (CDC) had reported influenza in 49 of the 50 states. However, only one state (Virginia) has had widespread influenza activity, 5 have had regional activity, and 10 have had local disease activity. Sporadic activity has been reported in 33 states, the District of Columbia, and Puerto Rico. But at this writing in early February, we're just now seeing a notable increase in influenza-like illnesses and culture documentation that both influenza A and B have arrived here in Kansas City.
This late start sends a clear message about prevention: It's not too late to vaccinate. All children aged 6 months and older now are recommended to receive influenza vaccination. But because infants younger than 6 months are not eligible for influenza vaccine and antiviral medications are not indicated for those younger than 1 year, a “cocoon” strategy is best for infants. This approach works by immunizing the persons most in contact with infants—mostly family members, but ideally also the day care personnel, babysitters, etc., thereby creating a “zone of protection” around the child.
The CDC's Advisory Committee on Immunization Practices (ACIP) is moving toward a universal recommendation for all persons over age 6 months to receive the influenza vaccine. Expect that recommendation to be made within the next year. In the meantime, recent data suggest that cross-protection and protection in general is likely to be superior with intranasal vaccine, compared with injected vaccine. Unfortunately, the intranasal vaccine (FluMist) is not approved for use in children under 2 years old or adults older than 50 years. I'd like all health care staff to be able to receive it, and I wish the ACIP would recommend its use in the 50-plus age group, despite current labeling.
To date it appears that this season's influenza vaccines match the circulating A strains, while the influenza B match may not be quite as good. However, it's still too early to predict for certain because the number of isolates is small and so far mostly from only three states.
With regard to influenza treatment, the circulating strains thus far are presenting us with a clinical conundrum: For the last 2 years, we've been told to stop using rimantadine and amantadine because they don't work on influenza A (they were never effective for influenza B), and to restrict antiviral therapy to two available products, oseltamivir and zanamivir. Now we find that we need to partially reverse course. This year, two-thirds of typed circulating strains are H1N1 strains that are resistant to oseltamivir but surprisingly susceptible to rimantadine/amantadine.
Of the strains currently circulating, one-quarter is influenza B and is still susceptible to oseltamivir and zanamivir. Less than 10% of all circulating strains have been H3N2, and these also are still susceptible to oseltamivir and zanamivir, but resistant to rimantadine/amantadine, similar to last year. So far, the proportions of types A vs. B in Kansas City have been the same as the proportions reported nationally by the CDC.
So here's how it could work clinically: If the patient presents within 48 hours of fever onset and a rapid antigen test shows influenza B, you can proceed as in the last 2 years and treat with oseltamivir or zanamivir.
But if it's influenza A, it gets tricky: About 90% of the influenza As—the H1N1s—will be susceptible to rimantadine and resistant to oseltamivir, but the reverse is true for the 10% or so that are H3N2s. So for influenza A, it seems reasonable to offer rimantadine but explain that there's a 10% chance it won't work. Amantadine also is an option, although it has more frequent and often more severe side effects.
If the patient desires 100% certainty, the CDC says to consider both antivirals—rimantadine plus oseltamivir. We don't have prospective controlled data for using these two together, because this particular problem previously was not on our radar screen. Doing so also doubles the cost of treatment.
And here's another odd twist: Zanamivir, the neuraminidase-inhibitor cousin of oseltamivir, is still active against all circulating strains we've seen so far, including those that are resistant to oseltamivir. The problem with zanamivir, though, is that it's not approved in children under 7 years of age. Also, it is administered via rotahaler (also called a diskhaler), which can be tricky to manipulate. But if your patient is skilled in or capable of using this device, zanamivir is another option.