So far, the current flu season in Puerto Rico has claimed 42,183 victims — more than 3,000 just in the past week, according to the CDC. By contrast, the entire 2024-2025 season had a total of 64,339 cases. Since flu seasons run from June to June, that means that with only half the season counted, this year’s flu season has already reached 2/3 of the total for last year. The cases in the past week have been more than double the weekly average from last year. Last year’s flu season across the United States was the worst in two decades, so an even worse season this year is very bad news. In fact, the Puerto Rico Department of Health has declared a flu epidemic.
Why is this year’s flu season worse than last year?
The strain of flu in Puerto Rico right now is caused by the influenza A(H3N2) virus. Influenza A tends to be more severe than Influenza B, but the strain being seen this year is an unusual mutation called “subclade K.” This variety has not been seen much in America, and it was not one of the strains used to develop this year’s flu vaccine.
Each year, scientists use information about previous cases to decide what strains of the flu to use in developing the new season’s vaccine. The information is likely to come from Australia, where the flu season begins earlier than in the Northern Hemisphere. Subclade K turned up in Europe after the vaccine had already been created, so it was not included. The current vaccine protects enough against Subclade K to make it less likely that vaccinated people will end up in the hospital or worse, but it protects at about 40% against that particular variety.
As a result, we are less likely to have natural immunity against this variation of the flu, sin ce most people have never had it before. Since this was not one of the strains used to develop the vaccine, even people who have been vaccinated are less likely to be protected. Also, since fewer people have been vaccinated this year, there is less chance of herd immunity.
What’s herd immunity?
Say you work in an office with a total of 10 people, including yourself. If one person has the flu and the rest are vaccinated, chances are good that no additional people will catch the flu. If 8 people are vaccinated, the one sick person may not come into contact with the one other unvaccinated person. There are 10 people, and the people in between the one with flu and the one other unvaccinated person improve that person’s chances of not coming into contact with the virus. In fact, if the rate of vaccination is high enough, the unvaccinated people are protected by those who got their shots.
In Puerto Rico during this flu season, just about 25% of adults got the flu vaccine — and in some age groups it was as few as 17%. That means that our office with ten workers could have 8 unvaccinated people. If one gets the flu, at least a few more unvaccinated people are almost sure to be in contact with her. They will not be protected by herd immunity because most of the herd will not be either immune or vaccinated.
Now what?
If you are not in Puerto Rico right now, this is not the time to travel there. If you are in Puerto Rico, consider taking up your COVID-era precautions:
- Stay home if you feel sick, and keep your children home when they are sick.
- Avoid indoor crowds.
- Keep social distance.
- Wash hands frequently.
- Avoid touching your face.
No matter where you are, get your flu shot. It is not too late — the vaccination takes effect within a few days.
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