Like many healthcare workers, I am required to get a flu shot by the end of October each year. Every year, I wait until the last day I can get vaccinated.
That’s because numerous studies have shown that the effectiveness of the flu shot, like that of the COVID-19 shot, diminishes significantly over time, especially in high-risk populations. After just a few weeks, antibodies produced in response to influenza vaccination reach their highest levels. You don’t want your antibodies to plateau in September and October and then drop as you approach the reliable winter flu peak.
Still, tens of millions of Americans heeded public health messages and started getting vaccinated as soon as possible, starting in September. Currently, one-quarter of older adults in the United States receive a flu shot each year. Their bodies are filled with new antibodies induced by the vaccine and are ready to defend against any influenza virus they encounter.
The problem is, there are no battles for them to fight.
Levels of seasonal influenza remain reliably low at this time of year in the United States. Levels may begin to increase in November, but in most years they only really increase in December or later. In the first week of last October, the number of weekly influenza-related hospitalizations fell by more than 44 times compared to the final peak, which occurred in late December, as expected.
Why doesn’t the Centers for Disease Control and Prevention recommend that people wait until late October or even early November each year?
CDC epidemiologists are chasing the wrong results. They seem to focus on one metric: vaccine uptake, the percentage of the population vaccinated against influenza each season. Instead, we need to focus on something else: the rate of avoidance of severe disease.
These two goals can conflict with each other. Public messages encouraging people to get vaccinated early may increase uptake rates, but may reduce overall protection effectiveness during the winter months when it is most needed.
One model that takes into account how vaccine effectiveness declines over time suggests that in years when influenza cases peak in January or February, the annual vaccination campaign may be delayed until September. It suggests that starting in October instead could reduce influenza-related hospitalizations by 3 to 4 percent. This may not seem like a big deal, but it could result in up to 10,000 fewer hospitalizations over the next few years.
The problem is vaccinating everyone in a short period of time. The benefits of improved efficacy may be negated by a reduction in the number of people vaccinated. That’s why the CDC has so far chosen not to delay its annual flu vaccination campaign.
There is a solution.
The CDC should start vaccinations in October instead of September, which with a concerted effort should be enough time for a successful vaccination campaign. The National Institutes of Health and other researchers should also begin studies to evaluate the effectiveness of two-dose influenza vaccines in high-risk populations, such as the elderly and immunocompromised. Doing so will better protect them throughout the flu season. Experience with Covid-19 vaccines shows that for high-risk populations, booster shots given just a few months apart are associated with reduced Covid-19-related hospitalizations in that population, and influenza We found that vaccinations are likely to be equally effective.
Scientists have tested two-dose versions of influenza vaccines in the past, but they tried the wrong regimen. Rather than testing two shots several months apart (for example, in October and January), the study evaluated a second shot given just a few weeks after the first. By then, antibodies are already so high that additional shots are unlikely to be very effective. bump. However, a second dose given several months later, after antibodies have waned, may prevent more hospitalizations and provide more protection. We need scientists to study whether this approach works in the real world.
Will it be difficult to convince people to get two shots when many won’t get one? Vaccine hesitancy is a problem, but older people and other high-risk people are consistently getting vaccinated. These people will benefit the most.
New data from the recently concluded influenza season in the Southern Hemisphere shows this year’s flu shot was 35 percent effective in preventing hospitalization. However, that number is an average. Effectiveness also varies by age and other risk factors, and probably also by when during the season the vaccine is given. It was 31 percent effective in older adults, but 59 percent effective in younger adults with comorbidities, another important target group.
What does this mean for those who already receive their annual flu vaccine? All is not lost. Many people, especially those who have a strong immune response to the vaccine, may have lasting immunity into the winter season. And for people who reliably get a flu shot each year (including older adults, who have by far the highest rates of annual flu shots), there may be cumulative benefits from regular vaccination. This may be due to long-term strengthening of the immune system, which may help prevent, or at least reduce the severity of, infections that worsen chronic diseases.
For people who haven’t yet gotten the flu vaccine, delaying it may work in their favor if they can still get it in the next month or so. Remember that vaccines only work when they are inside our bodies.
