Flu update: Just how bad has it been in King County | Public Health Insider

The following was written by Meredith Li-Wollmer for Public Health Insider:

Copious media coverage has described the unusually widespread and severe flu season nationally. But what has it been like in our region? We checked in with Dr. Jeff Duchin, Health Officer and Chief of Communicable Disease Epidemiology and Immunization, to find out the current status of flu locally.

What's happening now and how do you determine how bad a flu season is?

JD: Influenza-like illnesses remain significantly elevated in King County, but they have begun to fall in all age groups. However, the flu season is far from over with 6-8 weeks of elevated but decreasing flu activity likely remaining.

With respect to severity, there are several things to keep in mind. Regardless of how we classify each flu season, those people who do get the flu have a miserable illness, miss school and work and more enjoyable activities, many require health care visits or need hospitalization, and some will die. So from the individual's personal health perspective, there is really no such thing as a "mild" flu season.

To describe severity from the population health level, we look at the number of people who are seen in King County emergency departments with influenza-like illnesses. We get additional information about severity by tracking what proportion of these people is hospitalized, and through reports of influenza deaths from healthcare providers. It's important to note that reported influenza deaths are "the tip of the iceberg," and actual influenza-related deaths are likely 10-20 times higher than reported.

We also track how widespread the flu is through the number of laboratory tests for influenza that are ordered and are positive, and through reporting of outbreaks at longterm care facilities and other institutions.

Although absolute cutoffs that correspond to mild, moderate, and severe seasons are not available at this time, we compare all this information from the current season to past seasons to determine the relative severity. We're in the process of developing local cutoffs based on recent research from the CDC.

Got it. So what do the measures say about how this flu season compared to previous seasons?

The main flu strain that has been spreading has been H3N2, which is a strain that is generally more severe than others. This strain was also the predominant one last year, and that was the worst flu season locally in recent times. Our measures show that the current flu season in King County hasn't been as severe as the 2016-17 season or the 2014-15 season, another bad one. I think people tend to forget from one year to the next how severe our annual flu seasons can be.

If we look at two measures, the number of deaths from flu and the number of outbreaks in long-term care facilities, this year hasn't been as bad as last year. Twenty-two influenza deaths (all in adults) and 27 long-term care facility outbreaks have been reported this season (through February 3rd) compared to 53 deaths and 81 long-term care facility outbreaks last year at this time. There have been no pediatric deaths reported in King County in the past 5 seasons. Hospitalizations for persons with influenza-like illness seen in King County emergency departments are also at lower levels then last season and the 2014-15 season.

How effective has the flu vaccine been so far?

The flu vaccine is formulated to protect against multiple strains. The H3N2 strain has been circulating most widely, and it also caused severe outbreaks last year and in 2014-15. Influenza B is also circulating and tends to increase in the latter half of the flu season. Early estimates of influenza vaccine effectiveness from Canada show overall protection against any influenza virus infection of 42%, with low protection against the H3N2 strain (17%) and higher (55%) protection against type B influenza. In other words, more than half the vaccinated people who were exposed to an influenza B virus didn't get it, and 17% of the vaccinated people who were exposed to H3N2 didn't get it.

It's important to know that even in seasons when the vaccine provides relatively low protection against one of the influenza strains, it still can provide meaningful benefit in preventing hospitalizations and reducing the severity of symptoms and length of illness.

If the H3N2 part of the flu vaccine only provided low protection and that's the main flu virus circulating, why hasn't this season been worse in King County?

That's an interesting question. State and national level data may mask variability in influenza activity in smaller geographic areas such as the county level. But the county level may tell us something about influenza epidemiology and immunity. To-date, this year's outbreak locally has not been as severe or deadly as last year, particularly among older individuals. What might be reasons for that? Perhaps people who were sick with H3N2 last year had some protection against a similar H3N2 virus. It's an intriguing possibility. Also of interest is the potential impact of the severity and extent of the prior season's flu outbreak on vaccine effectiveness – an issue that has not been studied extensively.

So are you saying that last year's bad flu season may have been part of the reason why the flu wasn't worse locally this year?

It's possible, especially given that the outbreak so far has been largely due to a similar H3N2 flu strain, but we won't be able to fully characterize severity until we have all the data from this season. The flu season has many weeks left and in addition to the H3N2 virus that continues to circulate, influenza B may increase before this year's outbreak ends. Early estimates from Canada show the current vaccine provides protection against influenza infections overall although it is not performing well against the H3N2 strain. By the end of the season we should also know more about the true vaccine effectiveness.

For more information about flu: https://www.kingcounty.gov/depts/health/communicable-diseases/disease-control/influenza.aspx