The best time to time a flu shot

I wager on October myself.

The pharmacies are already advertising: Get your flu shot now.

Is this really the best time to get your flu shot when influenza rates are low, and the vaccine has a tendency to wear off?

The timing game

There is no one-size-fits-all answer. Personally, my plan is to wait until mid-October to get my annual flu shot. I’ll be keeping an eye on estimated flu rates in the U.S. and might move that target date forward or even a little back depending on how things are going.

I’m getting my updated Covid shot as soon as I can though, hopefully in early September if not sick before then.

Some information from UpToDate follows:

The timing of the annual flu season — as determined by elevated flu activity – varies from season to season. During most seasons, activity begins to increase in October, most often peaks between December and February, and can remain elevated into May.

It’s rare, but not impossible, to have influenza peaks in the early fall. Over the past 20 years, here is when the peaks have occurred, with February taking the gold medal:

Other medical experts generally recommend getting the annual flu shot in early fall, typically by the end of October. This timing is based on several factors:

• Waning protection: The flu vaccine's effectiveness can decrease over time, with studies suggesting a gradual decline in protection over 6 months.

• Peak flu season: In the Northern Hemisphere, flu activity typically peaks between December and February, although it can last as late as May.

• Immune response time: It takes about two weeks after vaccination for antibodies to develop and provide protection against the flu.

• Unique personal risks: Older adults (65+) and those with weakened immune systems might benefit from slightly later vaccination (e.g., in October) to ensure protection keeps up through the entire flu season.

Besides the flu shots, there are a lot of other shots vying to get in our arms, too. According to CDC and UpToDate, it is OK to give the flu shot with an updated Covid shot, or an RSV shot, or a pneumonia shot. However, if we are diligent and reliable with follow through, I like to spread mine out, with a couple weeks in between flu and Covid shots… especially this year since we are enduring a massive Covid wave right now. The sooner we can get updated JN.1 or KP.2 shots in the arms of people who have not had Covid during this wave the better.

Unlike Covid, U.S. flu activity is estimated to be very low right now. You can check in on this by following the weekly CDC influenza activity map. Of course, they use a wide array of red/green colors for the 1 out of 12 men who are red/green colorblind. No respect:

Overall efficacy

Depending on the year and how well matched the vaccine is, protection against infection ranges from 50 to 80%. But pooling all those years together we get the following averages:

• Vaccination is associated with a reduced incidence of contracting influenza:

  • In a 2018 meta-analysis including 52 trials and more than 80,000 healthy adults, influenza vaccination reduced the incidence of influenza from 2.3 to 0.9 percent, a 59% relative risk reduction, corresponding to a number needed to vaccinate to prevent one infection of 71 people [44]. However, it should be noted that the CDC estimates true infection rates of 5-20% of people contracting the flu in any given year.

  • In a 2018 meta-analysis including eight trials and more than 5000 adults ≥65 years, influenza vaccination reduced the incidence of influenza from 6 to 2.4 percent, for about a 58% reduction in cases [45].

• Among vaccinated individuals with breakthrough influenza virus infection, vaccination is associated with reduced mortality and attenuated disease severity [46]:

  • Among five observational studies of adults with influenza-associated hospitalization, vaccination was associated with 31 percent mortality reduction.

  • Among eight studies of adults with influenza-associated hospitalization, vaccination was associated with 26 percent reduction in odds of intensive care unit admission.

How long does the flu shot provide protection?

Some protection is durable, but the highest protection is in the first month or two.

Flu vaccine effectiveness wanes 9% every 28 days, starting 41 days post-vaccination in adults (but not in children, who were found to have stable protection throughout the whole season!), according to a study published this year in Eurosurveillance. The study went out to 5 months after flu vaccination and found that overall:

Our results were comparable with other studies examining protection by time since influenza vaccination. A study from Singapore (which has year-round influenza activity due to its tropical climate) found that the infection odds increased 1.07 times every 8 weeks since vaccination [8]. A study in the US found that the odds of influenza A(H3N2) infection increased 1.12 times every 14 days [16]. Another US study found that the odds of influenza increased 1.16 times every 28 days [9].

What’s in the flu shot this year?

Are they still quadrivalent (covering 4 strains)?

For the 2024-25 season, all influenza vaccines available for administration in the United States will be trivalent (covering three types), containing a representative strain of influenza A/H1N1, influenza A/H3N2, and influenza B/Victoria lineage.

Viruses of the influenza B/Yamagata lineage have not been identified globally since March 2020, and the WHO and other public health authorities have recommended that it be removed from the seasonal vaccine.

Masking, and the collective pain of temporary business/school closures and physical distancing, did apparently lead to the extinction of that one strain of influenza B/Yamagata.

Excellent. I never liked that virus anyway.

Less influenza illnesses, less heart attacks

Every case of flu prevented matters. Just like Covid, flu can do bad stuff under the hood. Did you know that undiagnosed influenza occurs in almost 10% of people admitted with heart attacks, especially during the flu season? I’m going to highlight a recent NEJM study about this in a future quick post, but here is the bottom line:

Results showed that the overall risk of heart attack was 6.16 times higher in the week after influenza infection. Notably, this risk was significantly higher (16.60 times!) for individuals without prior coronary artery disease.

So, does getting a flu shot reduce the above risk of cardiovascular complications? Yes. In a study published in 2020 looking at high risk people (defined as people over age 50 (yikes, that’s almost me!), HIV/AIDS patients, those residing in nursing homes, and people who are obese), getting a flu shot was associated with a:

• 28% reduced risk of heart attack

• 47% reduced risk of TIA

• 73% reduced risk of death.

I’ll stop there as I think you get the point, despite many people trivializing flu shots. But more “patient education” can be read here via UpToDate if you want more.

Conclusion

Apropos of absolutely nothing, here is an audio of some frogs called coquis, singing to each other on a rainy night in Yabucoa, Puerto Rico. I was there over spring break earlier this year 🐸

Again, apropos of nothing, except that I felt this post needed some levity and music.

Best of luck playing the flu shot game.

And don’t forget, if you qualify for a pneumonia shot against the bacteria Streptococcus pneumoniae, I previously wrote about how these can paradoxically also help reduce viral infections and complications.

The experts say that good flu shot bets are usually placed in October, but keep your eyes on this year’s actual flu trends. Spread the word and forward at will. And as they say, talk to your doctor ;)