This story has been updated for the 2019–2020 flu season.
The flu is a reliably serious public health problem. Last year’s flu season lasted 21 weeks — the longest in a decade — and caused more than 500,000 hospitalizations and at least 36,000 deaths, according to the Centers for Disease Control and Prevention. (Those numbers were even higher the previous year.)
Even so, a lot of people drag their feet when it comes to getting a flu shot. In a given year, only about 40 percent of adults get vaccinated. Dr. Pedro A. Piedra, a physician and researcher at the Vaccine Research Center, would like to see that number, called flu coverage, rise to 80 percent.
People cite all sorts of reasons for opting out of vaccination. Some worry they’ll get sick from the flu shot itself, while others distrust vaccines generally or believe the vaccine is pointless since it doesn’t protect against every strain of the flu. Still others assume that a flu-free past means a flu-free future. But in the vast majority of cases, experts say there’s no justification for riding out the flu season without protection.
“Last year we had an unusually long influenza season with two major overlapping outbreaks. The year before, we had an influenza season that caused very high attack rates, high hospitalizations and high mortality in both children and older adults,” says Piedra, who is also a professor of virology and microbiology at Baylor College of Medicine in Houston. “What will happen this year, we can’t predict. But I can say that the flu will circulate and will cause problems, and the best way to protect yourself from it is to be vaccinated.”
The flu shot: backstory and basics
The influenza virus was first isolated in a laboratory in the early 1930s, and a vaccine has been in use since the end of World War II. The flu isn’t a single virus: There are two main types of influenza, A and B, each with multiple strains. The diseases caused by all of these viral strains are collectively called “the flu,” and between 9.2 million and 35.6 million people get it every year. The vaccine helps the body produce an antibody response that prevents viral infection.
Early flu-shot guidelines focused on vaccinating high-risk populations, meaning children, the elderly and people with compromised immune systems. Once Medicare began covering flu shot costs for patients in 1993, vaccination became more widespread in the U.S. In 2010, the CDC began recommending flu shots for everyone 6 months old and up.
The disease primarily spreads from the mouth, nose and eyes when infected people talk, cough and sneeze. It’s also possible to pick up the flu if you touch a surface harboring the virus and then touch your face, although this mode of transmission is less common. Symptoms can include fever, body aches, fatigue and sometimes diarrhea and vomiting. Most people will only get mildly ill from the flu and won’t need medical care or antiviral drugs, says Bert Kelly, a spokesman for the CDC. But one person’s mild case of the flu can turn into another person’s serious illness. That’s why the CDC asks that anyone with the flu stay home, refrain from travel and avoid contact with other people except to get medical care.
These instructions might seem obvious, but in a 2018 consumer poll of more than 2,000 CVS customers, two-thirds of respondents said they’d go to work with the flu, either because they didn’t want to use paid time off or because they’d be expected to work through sickness. Respondents also said flu symptoms wouldn’t prevent them from going to the grocery store (28 percent), picking up children at school (12 percent) or taking public transportation (10 percent).
Why flu vaccines change every year
Influenza viruses mutate constantly, which is why vaccines need to be updated. Each year, researchers figure out the strains by analyzing data collected from early cases of the flu in a worldwide surveillance network. Once the virus is isolated in patients, doctors conduct testing to determine the virus subtype.
The next step entails even further testing to understand the composition of the major viruses circulating among flu-infected people. Once researchers have a best guess of which strains are most likely to spread, they develop vaccines anywhere from three to six months in advance of flu season. Each vaccine contains three or four different strains of the flu. When the vaccines match the types of flu going around, flu risk among the general population drops by 40 to 60 percent, according CDC research. But even if their best guess is wrong, and there isn’t a perfect match between the vaccines and the current flu strains, getting vaccinated still provides some protection against infection.
CDC research suggests that four strains are responsible for this year’s flu season: H1N1 (the strain behind the 2009 pandemic), H3N2, and two main lineages of the B virus. Vaccines for both H1N1and H3N2 have been updated since last year, while the B-virus vaccines are unchanged.
How the vaccine protects you (and everyone else)
It takes about two weeks for the flu vaccine to take effect. During that time, the body develops antibodies to fight the virus. Some people are more sensitive to the flu shot and might experience mild flu-like symptoms afterward, but the flu shot will not give you the flu. In addition, some people might experience soreness at the injection site, but it only lasts a day or two.
The CDC estimates that flu vaccines are about 30 percent to 60 percent effective in any given year. That might sound low, but there’s something else at play: herd immunity. Populations can collectively stop flu transmission if enough people are immune to infection.
“The definition of ‘enough,’ that magic number, is based on how transmissible the infection is,” says Stephen S. Morse, an epidemiologist at the Mailman School of Public Health at Columbia University. “There are some standard ways to model this mathematically.”
A 50-percent vaccination rate would be enough to stop flu transmission in a geographic area. By contrast, it would take a 95-percent vaccination rate to build herd immunity to measles, which is about 10 times as transmissible as the flu.
“Even if a vaccine is only partially effective,” Morse says, “it will still reduce transmission significantly, as well as protecting you.”
Does everyone get the same flu shot?
Most but not all flu vaccines are administered via injection. The reasons to get a particular type primarily come down to age and health status. It’s not your responsibility to choose the right flu vaccine; doctors, nurses, pharmacists and anyone else administering vaccines should collect enough information from you to make a selection on your behalf.
Here are the commonly available vaccines:
- Regular-dose flu shot (IIV): the traditional “flu shot,” an injected vaccine made from an inactivated (i.e., killed) flu virus; a number of manufacturers produce standard-dose flu shots. All regular-dose shots for the 2019-2020 season will be quadrivalent vaccines, which protect against four flu strains.
- High-dose flu shot: an injected, high-dose vaccine for people 65 and older.
- Flu vaccine with adjuvant: Also only for people over 65, a flu vaccine with adjuvant added to it helps spur a stronger immune response to the vaccination for weaker immune systems.
- Egg-free flu shot (RIV): an injected quadrivalent vaccine made from recombinant DNA.
- Nasal spray (LAIV): a nasal-spray quadrivalent vaccine made from a live, weakened virus.
Who gets which vaccines?
Many people can receive multiple types and brands of flu vaccine, and the CDC says there’s no reason to seek out one vaccine over another. With that said, age and health issues dictate which types are appropriate.
For nonpregnant people ages 2 to 49, the CDC now recommends the nasal spray vaccine in addition to standard-dose flu shots. The spray was unavailable for the past two years due to uncertainty over its effectiveness against the H1N1 virus, but a tweaked version is back in play. Last year, the American Academy of Pediatrics expressed a preference for flu shots over the spray for children, but did not make such a distinction for the 2019-2020 flu season. Whichever CDC-approved version pediatricians recommend for kids is fine.
Starting at age 50, adults should get shots, not the spray. People 65 and older should get one of the higher-dose options mentioned above. A high-dose shot is also typically recommended for organ transplant patients. But it’s important to get vaccinated even if only standard-dose shots are available in your area. (I.e., something is better than nothing.)
Pregnant and nursing mothers should get vaccinated with the standard-dose shot rather than the nasal spray. The CDC also recommends shots over the spray for children taking aspirin (for any reason) and people with compromised immune systems who’ve recently taken antiviral drugs.
People with egg allergies can safely get the egg-free shot. For those with milder egg allergies (nothing worse than hives), the egg-free shot may not be necessary. Check with your doctor and the CDC guidelines for more information.
Should anyone not get a flu vaccine?
The vast majority of people can (and should) get a flu vaccine. The CDC notes two exceptions:
- People with a history of severe allergic reactions to a flu vaccine or a vaccine component other than egg.
- Some but not all people who’ve previously developed Guillain-Barré syndrome within six weeks after receiving a flu vaccine.
These two populations shouldn’t write off flu protection altogether. Doctors may opt to use preventive antiviral drugs instead. Aside from these specific instances, the CDC’s Kelly says that everyone over 6 months old should get the flu vaccine, particularly members of high-risk populations, including children under 5, adults 65 and older, pregnant women, residents of nursing homes or long-term care facilities, people with severe obesity, American Indians and Alaska Natives, and people who have certain medical conditions, such as chronic asthma or heart disease.
How long is flu season?
So when should you get the flu vaccine?
As a rule, Piedra says it makes sense to think of back-to-school season as a flu-shot reminder. The CDC’s standard recommendation is to get vaccinated by the end of October, and flu shots generally become available around the first week of August. While Piedra says most people don’t need to worry about getting vaccinated too early, the CDC issued a warning this year regarding summertime flu-shots: Vaccinating in July or August might reduce people’s ability to fend off the virus for the length of the flu season, particularly if they’re older.
Children between the ages of 6 months and 8 years old getting their first flu shot, however, are supposed to receive two vaccine doses at least four weeks apart, so they should get the first dose as soon as possible to fit in the second one by the end of October, the CDC says. Kids in this age group who’ve received flu vaccines before only need one dose.
“I think it’s better to get one a little too early than too late,” Morse says. “There isn’t solid evidence about when it’s best to get a flu shot, except that you definitely want to get it at least two weeks before you might be exposed to the flu.”
But if you don’t get vaccinated by late October, don’t write it off for the season. Getting the vaccine in late winter will confer protection through spring, when the flu is still being passed around.
Where should you get vaccinated?
Primary care doctors generally administer flu shots in their offices, and many employers, schools and drug stores offer them as well. If you need help finding a shot spot, the CDC recommends vaccinefinder.org.
Is there any chance the vaccine will run out?
How much does flu vaccination cost?
Vaccination costs range from nothing to $70, depending on the type of vaccine your doctor recommends and your insurance coverage. High-dose shots for the elderly tend to be the most expensive, but Medicare should cover them. Affordable Care Act health plans should cover flu vaccination without a co-pay, but check with your carrier. CVS, for example, offers shots for $40 for the standard dose and $70 for seniors.
The future of the flu shot
A study is underway to find out whether a topical cream that treats genital warts and skin cancer might be able to boost immune responses against the flu. If so, it could help stretch the supply of vaccines for H5N1 (avian flu) and allow more people to get vaccinated during an outbreak. Researchers are also working on the phase 2 clinical trial for an experimental universal flu vaccine.
It will take at least one to two years for either to be publicly available, but for medical professionals and public health officials, anything that will increase flu coverage is a positive step forward.
“There are always barriers to getting enough people vaccinated, and the need for an annual vaccination is one barrier,” Piedra says. “It would be nice if we could produce a longer duration of immunity.”