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Influenza (flu) is a viral respiratory infection that affects millions of people each year. As we head into flu season in the United States during the COVID-19 pandemic, it’s important to know what to expect and how to prevent it.
Every year, flu vaccines are developed to protect against the most commonly circulating strains. Receiving the seasonal flu vaccine is the best way to protect yourself against becoming ill with the flu.
But how does the vaccine work? How long does it last, and when’s the best time to get it? Here’s what you need to know.
Development of the seasonal flu vaccine actually begins many months ahead of flu season. The viruses used in the vaccine are based on extensive research and surveillance into which strains will be most common during the upcoming season.
Seasonal flu vaccines protect against two types of influenza viruses: influenza A and influenza B. They can also be either trivalent or quadrivalent.
The trivalent vaccine protects against three flu viruses: two influenza A viruses and an influenza B virus.
The quadrivalent vaccine protects against the same three viruses as the trivalent vaccine, but it also includes an additional influenza B virus.
Once you receive your flu shot, it takes 2 weeks for your body to develop antibodies that provide protection.
It’s important to remember that during this period, you’re still vulnerable to becoming ill with the flu.
During that time, you should be extra careful to:
- practice good hygiene
- avoid touching your nose or mouth whenever possible
- avoid crowds if flu is circulating in your community
These precautions are exponentially more important while COVID-19 is still a factor. You can develop the flu along with other respiratory infections, so protecting yourself and others is important.
Your body’s immunity to the flu decreases over time. This is true whether you’ve had a vaccination or a flu infection.
Additionally, influenza viruses are constantly changing. Because of this, a vaccine from the previous flu season may not protect you through an upcoming flu season.
Generally speaking, receiving the seasonal influenza vaccine should help to protect you for the duration of the current flu season.
You’ll need to receive a seasonal influenza vaccine every year in order to have the best protection against influenza viruses.
The flu vaccine is produced by a number of private manufacturers and typically begins to ship to healthcare providers in August. However, there’s some evidence that it may not be advantageous to receive your vaccine this early.
A 2017 study indicated that maximum immunity is achieved shortly following vaccination and decreases with each passing month. Therefore, if you get your vaccine in August, you may be more susceptible to infection late in the flu season, around February or March.
The Centers for Disease Control and Prevention (CDC) recommends getting the flu vaccine before influenza activity begins to pick up within your community, ideally by the end of October.
If you receive your vaccine later, don’t worry. Late vaccination can still provide adequate protection, as influenza can circulate within your community through March or even later.
The flu shot is made with an inactivated virus, which means you can’t develop the flu from the seasonal flu vaccine. But there are several side effects that you may experience after receiving it.
Side effects from the flu shot are typically mild and only last a few days.
Flu vaccine side effects can include:
- redness, swelling, or soreness at the injection site
- low-grade fever
- general aches and pains
Influenza viruses are constantly changing and evolving rapidly. Circulating influenza viruses can mutate from one season to the next.
Researchers need to select the specific influenza viruses to include in the vaccine many months before flu season begins. This means what’s in the vaccine may not always match what’s actually circulating during flu season. This can decrease the effectiveness of the seasonal flu vaccine.
Age can also play a role in vaccine efficacy because your immune system tends to become weaker as you age. The Food and Drug Administration (FDA) has approved a high-dose flu vaccine (Fluzone High-Dose) for people 65 and older.
The higher dose is aimed at providing a better immune response and therefore better protection within this age group. Multiple studies have shown increased effectiveness for those over 65 with the high-dose vaccine.
The CDC also recommends that some children between the ages of 6 months and 8 years receive two doses of the influenza vaccine during the first season in which they’re vaccinated in order to have sufficient protection.
It’s still possible to get the flu after being vaccinated, but research has shown that the illness may be less severe and that people who receive a flu shot may be less likely to be admitted to the hospital if they get the flu.
People over 6 months of age should receive the flu shot each year.
It’s particularly important for people who are at an increased risk for flu-related complications to be vaccinated.
This includes:
- people over 50
- anyone with chronic medical conditions
- people with weakened immune systems
- children between 6 months and 5 years of age
- people 18 and under who receive aspirin therapy
- pregnant women and women up to 2 weeks after pregnancy
- people whose body mass index is 40 or higher
- American Indians or Alaska Natives
- healthcare workers
- anyone living or working in a nursing home or chronic care facility
- caregivers of any of the above
Children under 6 months of age shouldn’t receive the influenza vaccine. To protect these children from potential exposure to the virus, all family members or caregivers should be vaccinated.
This is called herd immunity and will help protect those who can’t receive the vaccine.
Additionally, if you’re currently sick with an acute illness, you may need to wait until you’re better to receive the vaccine.
Before you’re vaccinated, you should let your doctor know if you’ve had:
These factors may indicate that you should not get the flu shot. But check with your doctor to see what they recommend.
Many flu shots contain a small amount of egg protein. If you have a history of egg allergies, talk with your doctor about receiving the flu shot.
Influenza viruses cause seasonal epidemics of respiratory illness every year and this year is particularly dangerous due to the ongoing COVID-19 pandemic. While some people might experience mild illness, others (especially certain high-risk groups) may experience a more serious infection requiring hospitalization.
Getting your flu shot each year is the best way to decrease your chances of getting sick with the flu. Additionally, when more people receive the flu vaccine, the virus is less able to circulate in the community.
You should aim to receive your flu shot every fall before influenza virus activity begins to pick up within your area.
If you experience any symptoms of a cold or flu, it’s important to avoid contact with others and get tested for the flu and COVID-19.
The annual influenza vaccine saves lives and spares many people from severe disease, which is why governments and employers promote and subsidize its use. But it's hardly an ideal vaccine, offering so-so protection that wears off rapidly. A new, one-of-its-kind study, published today in Science, helps explain those shortcomings: A key cell type hidden in bone marrow that quickly kicks into activity after vaccination fades within a few months, researchers found. The discovery could lead to new strategies to increase the vaccine's durability.
The best vaccines—such as the ones for measles, rubella, and diphtheria—provide almost 100% protection for life. Flu vaccines, however, often don't exactly match the rapidly evolving influenza virus, so their effectiveness changes each year: In the United States between 2009 and 2019, it ranged from a low of 19% to a high of 60%. And protection wanes quickly: If you live in a temperate region of the world and receive the shot in the early fall, immunity can disappear before the end of that winter.
To better understand the durability problem, Rafi Ahmed, an immunologist at Emory University School of Medicine, homed in on a type of B cell that resides in the bone marrow and whose role Ahmed helped uncover in 1996. B cells make antibodies that can attach to and disable viruses. Ahmed focused on a type of B cell called bone marrow plasma cells (BMPCs), which continuously produce antibodies after an infection or vaccination. So-called memory B cells also produce antibodies and are created the same way, but in contrast to BMPCs, they do not steadily pump out the protective proteins. Instead, as their name implies, memory B cells that are trained to recognize a specific virus kick into gear only when they're re-exposed to it. It takes them several days after an infection to produce high levels of antibodies—a disadvantage in influenza, which can cause disease rapidly.
To the surprise and disbelief of many, Ahmed's group showed in 1996 that some BMPCs can live for many years, meaning they could, in theory, confer long-lasting immunity. Whether influenza vaccines trigger high levels of BMPCs and if so, whether the cells are the long-lived variety was a mystery, however.
Ahmed and colleagues repeatedly examined the bone marrow and blood of 53 volunteers aged between 20 and 45 years old in the weeks and months before and after they received influenza vaccines. (Some people participated over more than one flu season.) The study was no fun for the participants: Removing fluid from within a bone is a challenging and painful procedure that involves piercing the pelvic bone with a special needle. "The logistics … were very difficult, and I think nobody will ever try to do the same thing again," Ahmed says.
Rino Rappuoli, chief scientist at GlaxoSmithKline Vaccines, says he knows of no other study that sampled bone marrow for vaccine research. "Rafi's work is great and pioneering," Rappuoli says.
The researchers found spikes of BMPCs specific for influenza 4 weeks after immunization. But after 1 year, the new cells were virtually gone. Rappuoli and others aren't particularly surprised by this but welcome the evidence. "This finding tracks nicely with the observed rapidly waning [blood] antibody titers and decreasing protection in humans after getting the flu vaccine," says Adam Wheatley, an immunologist at the University of Melbourne. "It's a really nice piece of work."
The study "helps define the landscape" of the flu vaccine's lousy durability, says Mark Slifka, an immunologist at Oregon National Primate Research Center who earned his Ph.D. with Ahmed more than 20 years ago but was not involved with this work. "They chipped away at the stone in terms of understanding why the immune response is short-lived," Slifka says.
But Slifka thinks the BMPC population stimulated by vaccines likely has a small proportion of long-lived cells, undetected in this study, that could offer more enduring protection. The way to boost their presence is to goose the system so that it makes more BMPCs overall, he says. One possible way to do this is with adjuvants, additives to vaccines that act as irritants, ramping up the immune response. It also may help to increase the amount of viral proteins in the vaccines, he says.
The first influenza vaccines, developed in the 1940s, used adjuvants. They contained killed flu viruses mixed it with a water-in-oil emulsion called "incomplete Freund's." But the adjuvant caused ulcers at the injection site, so it was dropped from later vaccines. To further reduce unwanted reactions, researchers also stopped injecting the entire killed virus, replacing it with only the surface proteins from the virus. The resultant vaccines had fewer viral proteins and no immune-boosting agents. These vaccines, used widely today, cause far fewer side effects—but they came at a steep cost, says Slifka, who last year published a review article that hammered in these points. "We've damaged the immunogenicity and the durability of the response."
But for the past 2 decades, improved adjuvants have found their way into licensed vaccines. A revamped influenza vaccine that has an oil-in-water adjuvant—the water shields the oil and makes it safer—has been used in Italy since 1997 and was approved by European and U.S. regulators in 2000 and 2015, respectively. But whether it's able to trigger long-lasting BMPCs is unclear. No one in Ahmed's study received this product—when the project began, it wasn't even licensed in the United States—which is "a pity," Rappuoli says.
"It's totally crazy" that most commonly used influenza vaccines don't include an adjuvant, Ahmed says. "I'm hoping that things will change in the influenza vaccine world, and 10 years from now, you will not be getting any nonadjuvanted vaccines. This has been going on for years. It's hard to change the industry."