Late in 2014, we got news that the most prevalent circulating influenza virus strain this season, H3N2 (A/Switzerland/9715293/2013), was antigenically different from the H3N2 strain that was used to produce this year’s flu vaccine.
Depending on the source from which one first heard this news, the sentiments ranged from the cynical — “the vaccine is totally ineffective” and “doesn’t work” — to the more optimistic — “even if it doesn’t prevent the flu, it will most likely reduce the risk and/or severity of flu-related symptoms and complications.”
Who’s right? The “doom and gloom” people or the “every cloud has a silver lining” camp? Well, as with everything in life, the truth lies somewhere in between.
First and foremost, there is no question that the influenza vaccine does decrease the chance of getting the flu (according to the Centers for Disease Control and Prevention, Cochrane Reviews and many others). But decreasing the chance of getting the flu is not the same as preventing it in everyone.
Therein lies the rub, especially for the flu vaccine. It’s simply one of the worst in our vaccine arsenal in terms of effectiveness and year-to-year consistency of protection. The CDC reports that over the past decade, flu vaccines have ranged in effectiveness from 10% to 60%. On average, that’s not much better than flipping a coin in terms of whether it will work or not for any given individual.
According to the CDC, last year’s vaccine — considered a good match to the circulating viruses — was in that upper range (50% to 60%) of effectiveness for children and adults under age 65. For older adults, as always, it was less effective — only about 39%. Put in a different perspective, even with a well-matched vaccine, 40% to 60% of people receiving it were still at risk for catching the flu anyway. In the end, 2013-2014 was considered a moderate flu season.
As you no doubt know, what seems to makes this year’s flu season and vaccine potentially more concerning are the following:
• The mismatch has left the vaccine less than half as effective (23%) as last year’s, when averaged across all age groups.
• The current season has been dominated by the more virulent H3N2 viruses, which over the past decade have been associated with the highest number of hospitalizations and deaths. In general, flu vaccine effectiveness is not as high in an H3N2 season as in other seasons.
• Merely a decade ago, there was a perfect storm of an H3N2 viral strain and a mismatched vaccine in the 2003-2004 flu season that resulted in nearly 49,000 flu-related deaths, the highest over the past three decades.
So has this been a worse flu season so far? As of this writing, the CDC flu surveillance website showed that this season’s peak has passed (it was the last week of December) and that one measure, the weekly percentage of outpatient visits for influenza-like illness (ILI), was a little higher (+1.4%) compared with last year.
This year, however, has followed almost the same time course and peak ILI numbers as the 2012-2013 flu season (the two graphs are virtually congruent). Furthermore, the percentage of all deaths due to pneumonia and influenza has been relatively similar for the past three years and doesn’t appear to be higher this year (so far). Of course, influenza-related pneumonia deaths are a very small percentage of deaths associated with the infection.
At any rate, taken together, it would appear on the surface that the mismatched vaccine and the “virulent” H3N2 prevalence have not been as bad as some believed they would be. Maybe the vaccine wasn’t totally ineffective. However, there is one statistic that does stand out: laboratory-confirmed hospitalizations. Last year at this time, overall (newborns through elderly) there were about 24 hospitalizations per 100,000 population. Two years ago that number was 33 per 100,000, and this year it is currently 44 per 100,000. As expected, the largest population for flu-related hospitalized is seniors.
Therefore, despite the fact that the percentages of ILIs and deaths due to pneumonia and influenza are about the same for this season and that of two years ago, the current flu season has witnessed almost a 50% increase in the number of hospitalized elderly (216 per 100,000 versus 144 per 100,000). As a note, the 2012-2013 season was also predominated by H3N2 viruses while the vaccine was reported to be roughly 56% effective.
So could this year’s increase in hospitalizations be totally blamed on the mismatched, 23% effective vaccine? Or could it be due to the prevalence of an even more virulent H3N2 virus? Or are both to blame? Who knows? It will be interesting to see, when the mortality statistics are tallied and reported for this flu season, if there were more deaths. But even then, it will be hard to determine the real culprit: the mismatched vaccine, this particular H3N2 virus or a host of other factors.
Here’s what we do and don’t know:
• Laboratory tests of vaccine match aren’t perfect predictors of what to expect in terms of how well a vaccine will work. A bad match, as determined by current vaccine efficacy and effectiveness methodologies, doesn’t necessarily mean a vaccine won’t work to provide protection. Conversely, even when there’s a good match, it doesn’t necessarily mean it will be protective for everyone.
• What is not known with certainty (even though it’s bantered about in the layman’s and medical literature and by word of mouth) is whether a flu vaccine that doesn’t actually prevent the infection is likely to reduce the severity of the flu when one does get it. Although there are studies and reports from the CDC and others suggesting that this may be the case, it has not been proven absolutely. In fact, there are other studies stating that there really isn’t any conclusive evidence showing that there is a “lessening” of symptoms or complications regardless of whether the vaccine is a good or bad match.
The bottom line? We really have a lot to do going forward to truly understand how to make more effective and consistent influenza vaccines. In the meantime, based on safety profile, simplicity, low cost and wide availability, it really comes down to an individual’s choice of having some protection, even if it’s just 23%. Sometimes that’s the best we’ve got. But it’s better than no protection.
ALAN AGINS is the president of PRN Associates Ltd., which specializes in continuing education in pharmacology, and a speaker for pharmacy e-learning provider RxSchool.