Around the world, more than 100 countries have influenza centers that monitor flu-like illnesses. The centers test samples from patients to determine whether an illness is caused by an influenza virus, and if so, which type.
Those centers report data to a handful of WHO sites around the world, and those sites in turn track which strains are circulating. In February, the centers’ directors and other experts meet to prepare for flu season in the northern hemisphere.
“What’s circulating in the southern hemisphere (mainly H3N2 in late 2017) kind of gives us a determination of what we might expect to see in our season,” Stated Kayla DeBusk, a vaccine accountability specialist with the Spokane Regional Health District. .
Contrary to popular belief, the effectiveness of the vaccine isn’t usually determined by health departments selecting the right flu strains. More often, it depends on whether scientists have a similar virus mapped out well enough to produce vaccines for it.
“We don’t have a good virus match” for some strains, DeBusk said. Typically, a vaccine strain of the flu is about a 40 to 60 percent genetic match to the virus it’s supposed to provide immunity for.
Vaccines are more effective in years when the main types of flu have been seen before and already have effective vaccine strains. Strains for vaccines are grown in hen’s eggs.
Some people believe flu vaccines are rushed or “untested” because they change composition every year, but DeBusk said that’s far from the truth.
“We’ve been doing this method of manufacturing flu vaccines for about 70 years,” she said.
Swine flu, also known as the H1N1 virus, is circulating again this season and has been included in this year’s vaccine, though the variant is slightly different from the one included in last year’s.
All vaccines protect against two types of influenza A, which can be transmitted from animals. Those viruses, including H1N1, make up most cases during the early part of flu season. Influenza B strains, which only affect humans, tend to pick up later in the season.
Quadrivalent vaccines include two types of influenza B, while trivalent vaccines have just one.
This year's trivalent vaccine will contain:
- an A/Michigan/45/2015 (H1N1)pdm09-like virus (updated)
- an A/Hong Kong/4801/2014 (H3N2)-like virus
- a B/Brisbane/60/2008-like (B/Victoria lineage) virus