How long do flu shot side effects last

Every year, as brisk fall winds sweep in, so does the annual flu season. The exact timing of the onslaught can vary, but cases of influenza typically start to ramp up in October and peak between December and February. This year there are already signs that, after two years of relative quiet, the flu may be about to come raging back. Many of the protective measures adopted against Covid-19 that also helped keep the flu in check have been loosened: People are traveling more and wearing masks less. And the fact that the Southern Hemisphere has already seen a rough flu season can offer some insight into how things may play out in the Northern Hemisphere.

The best way to protect yourself and your family against the flu is to be vaccinated. Yet many people hesitate to receive their shots. We asked experts to weigh in on what you need to know about the vaccine, its side effects and more.

How effective is the flu vaccine?

Flu vaccines are updated every year based on what experts learn from previous seasons, influenza patterns in other parts of the world and estimates of how the virus might change. On average, flu vaccines help reduce the number of people who get sick by 40 percent to 60 percent, said Emily Martin, an associate professor of epidemiology at the University of Michigan School of Public Health. That means that the vaccine can help prevent millions of influenza illnesses, as well as thousands of hospitalizations and deaths, per year. According to the Centers for Disease Control and Prevention, the flu causes roughly nine million to 40 million illnesses and 12,000 to 52,000 deaths annually.

In general, the vaccines tend to work better against illnesses caused by a strain of virus known as influenza B as well as by a subset of influenza A viruses, Dr. Martin said. The shots are less effective against a different subset of influenza A viruses that go by the scientific name H3N2. That’s because the H3N2 viruses mutate much faster and scientists’ predictions for how the vaccine can protect against these changes don’t always match, Dr. Martin said.

Even if you do get sick after receiving a vaccine, the shot can reduce the severity of illness. “That is really what we want to do: keep people out of hospitals, living normal lives,” said Dr. H. Keipp Talbot, an associate professor of medicine at Vanderbilt University Medical Center.

Studies also suggest that the flu vaccine may help reduce community transmission — meaning that vaccinated people may be less likely to transmit the virus to others, even if they’re infected. That is why the vaccine is an effective way to protect not only yourself but also your family members and people around you who may be more susceptible to the flu, Dr. Talbot said.

When is the best time to get a flu shot? How long will its protection last?

Immunity against the flu tends to wane over the course of a season. You have higher protection a couple of weeks after receiving the shot, compared with four or five months later, so it is a good idea to schedule your vaccine appointment close to the beginning of flu season, and not too early, Dr. Martin said. “I tend to get vaccinated in October so my antibodies are ramped up by the time holiday travel begins,” she said.

Some people wait longer, until November or December, especially if they are keeping a close eye on cases. But experts agree that it is important to receive the vaccine before cases start to surge. Your body needs at least two weeks after the shot to ramp up its defenses against the flu. People who are more susceptible to severe flu — especially older adults, pregnant women and very young children — should not delay their shots.

How much does it cost, and where can I get it?

Most insurers cover the cost of flu vaccines as part of preventive care. People who are 65 or older enrolled in Medicare Part B plans and most people on Medicaid can also receive their annual flu shots at no personal cost. And those without any insurance can access free or low-cost flu shots through state health departments, employer vouchers or federal initiatives like the Vaccines for Children Program.

You can use the C.D.C.’s vaccine portal to find clinics, pharmacies and other locations that offer flu vaccines near you.

Should seniors, pregnant women and young children get the flu shot?

People who are 65 or older are at high risk of developing serious complications from the flu, including pneumonia and inflammation of the lungs, that can cause difficulty breathing and lead to hospitalization and the need to be put on a ventilator. An influenza infection can also increase the risk of heart attack and stroke in older adults, said Dr. Tara Vijayan, an infectious disease doctor at the David Geffen School of Medicine at the University of California, Los Angeles. “As we get older, we have a natural decline in our immune responses,” she said.

Older people also tend to respond more poorly to vaccination compared with younger adults. That’s why the C.D.C.’s Advisory Committee on Immunization Practices updated its recommendation this year for people who are 65 or older to receive one of three flu vaccines specially designed to better activate older immune systems against the flu. Some of these vaccines have been around for several years before the recommendation was formalized. One option is the Fluzone High-Dose Quadrivalent vaccine, which contains four times the dose of a standard flu shot. Another option is the Flublok Quadrivalent vaccine, which is created from a small piece of genetic material from the virus. The third option is the Fluad Quadrivalent vaccine, which is manufactured with an adjuvant, a substance that enhances the body’s immune response. Many doctors and pharmacists are proactively offering these options for older adults, but it is still a good idea to check when you go for your shot, since this is the first year of the new recommendation, Dr. Martin said.

The C.D.C. and Advisory Committee on Immunization Practices recommend that pregnant women are vaccinated against the flu to protect themselves and their fetuses. Just as the body experiences immune decline while it ages, it also tones down its immune responses during pregnancy, Dr. Talbot said. And the diaphragm, a muscle below the lungs, tends to move higher to accommodate the growing fetus, changing the way pregnant women breathe and making them more susceptible to respiratory infections.

While postpartum women, even if they are breastfeeding, can receive any flu vaccine approved for their age, pregnant women should avoid the FluMist nasal spray vaccine, which contains live virus. This is because the virus in the vaccine could theoretically cross the placenta and infect the fetus, even though it has been “attenuated” or weakened, Dr. Talbot said. The vaccines available to pregnant women contain inactivated or dead virus, or just a small piece of the virus that cannot cause infection.

Children become eligible for flu shots at 6 months old. The very first time they receive a flu vaccine, they need to get two doses, four weeks apart. After that, they can get one flu shot a year, Dr. Talbot said. Children who are older than 2 have the option of getting the FluMist nasal vaccine if they are afraid of needles.

What are the side effects of the flu vaccine?

All flu vaccines have a good safety record. The side effects tend to be mild and go away in 24 hours to 48 hours, Dr. Martin said. You may experience some soreness or slight swelling around the injection site. Other common side effects include a general feeling of malaise, headaches, muscle aches and nausea.

It is a common misconception that receiving a vaccine can give you the flu, Dr. Vijayan said. But the vaccines are designed so that the flu virus in them is either inactivated or changed, and for this reason they cannot make you sick.

Because it takes two weeks to build up protection from the vaccine, it is possible that you might catch the flu from someone else in that time frame. Or you may develop a slight fever in the day or two after your shot as a side effect of the injection.

“Any time your immune system is revved up, you may feel a little bit tired, you may have some muscle aches, and you may even get a little bit of a fever,” Dr. Vijayan said. “Those are normal things to expect. That’s a sign that your immune system is preparing to fight the real deal.”