LJUBLJANA, SLOVENIA – Don’t hold your breath waiting for a substantially better, more reliably effective influenza vaccine.
That was a key cautionary message provided by vaccine expert Edward A. Belongia, MD, at the annual meeting of the European Society for Paediatric Infectious Diseases.
The effectiveness of seasonal influenza vaccine varies from 10% to 60% year by year, leaving enormous room for improvement. But many obstacles exist to developing a more consistent and reliably effective version of the seasonal influenza vaccine. And the lofty goal of creating a universal vaccine is even more ambitious, although the National Institute of Allergy and Infectious Diseases has declared it to be a top priority and mapped out a strategic plan for getting there ().
“Ultimately the Holy Grail is a universal flu vaccine that would provide pan-A and pan-B protection that would last for more than 1 year, with protection against avian and pandemic viruses, and would work for both children and adults. We are nowhere near that. Every 5 years someone says we’re 5 years away, and then 5 years go by and we’re still 5 years away. So I’m not making any predictions on that,” said Dr. Belongia, director of the Center for Clinical Epidemiology and Population Health at the Marshfield (Wisc.) Clinic Research Institute, which is part of the U.S. Influenza Vaccine Effectiveness Network.
One of the big problems in creating a more effective flu vaccine, particularly for children, is the H3N2 virus subtype. Dr. Belongia was first author of a systematic review and meta-analysis of studies of more than a dozen recent flu seasons showing that although vaccine effectiveness against H3N2 varied widely from year to year, it was consistently lower than against influenza type B and H1N1 ().
And that’s especially true in children and adolescents. Notably, in the 2014-2015 U.S. flu season, vaccine effectiveness against H3N2 in children aged 6 months to 8 years was low at 23%, but shockingly lower at a mere 7% in the 9- to 17-year-olds. Whereas in the 2017-2018 season, vaccine effectiveness against H3N2 in the 9- to 17-year-olds jumped to 46% while remaining low but consistent at 22% in the younger children.
“We see a very different age pattern here for the older children compared to the younger children, and quite frankly we don’t really understand what’s doing this,” said Dr. Belongia.
What is well understood, however, is that the problematic performance of influenza vaccines when it comes to protecting against H3N2 is a complicated matter stemming from three sources: the virus itself; the current egg-based vaccine manufacturing methodology, which is now 7 decades old; and host factors.
That troublesome H3N2 virus
Antigenic evolution of the H3N2 virus occurs at a 5- to 6-fold higher rate than for influenza B virus and roughly 17-fold faster than for H1N1. That high mutation rate makes for a moving target that’s a real problem when trying to keep a vaccine current. Also, the globular head of the virus is prone to glycosylation, which enables the virus to evade immune detection.