Answers to the most pressing questions about the influenza virus, flu vaccine, and everything else you should know

Last year’s flu season was a doozy. According to the Centers for Disease Control and Prevention (CDC), the 2018–19 flu season lasted 21 weeks — making it the longest in a decade. Already this year, the flu has claimed at least one life — a 4-year-old living in California — which has led some public health officials to state that this year’s season may, too, be a rough one.

“The flu is hard to forecast, but already we’ve been seeing scattered reports of flu,” says Stephen Morse, a flu expert and professor of epidemiology at Columbia University’s Mailman School of Public Health. That could mean that this year’s season will be bad, he says, or it could mean that the season will peak early and quickly burn itself out. Whether the 2019–20 flu season turns out to be merciless or mild, Morse says the early emergence of the flu should lead people to get their flu shot sooner rather than later — and preferably by the end of October.

While the CDC’s annual influenza stats are approximations, the flu infects some 30 million or more Americans in a typical year. In bad years, such as the 2017–18 season, the flu infected nearly 50 million people and caused as many as 79,000 deaths. Experts say those figures could be meaningfully reduced if more people got the flu vaccine. Morse mentions a phenomenon known as “herd immunity,” which refers to a sort of vaccination tipping point at which an infection can’t find enough vulnerable hosts to effectively disperse. “If enough of the population is not susceptible, the infection won’t spread,” he says.

While there’s no guarantee that this year’s flu shot will offer perfect protection from the season’s circulating flu strains—more on that below—the flu vaccine typically cuts a person’s risks by 40% to 60%. If every American were vaccinated, rates of flu would plummet.

The Virus

What is influenza, exactly?

It’s a common, highly contagious type of viral respiratory infection. The virus that causes influenza comes in four different types or “lineages,” which are known as types A, B, C, and D.

Type D flu viruses don’t make people sick (they usually affect cattle) and type C’s tend to cause only mild infections. And so experts usually focus their attention on type A and B flu viruses, which are the two that account for the seasonal rise and fall in flu infections. Each of these influenza types can be further broken down into different subtypes and “strains.” (For example, H1N1 and H3N2 are common strains of influenza type A.)

How can you tell whether you have the flu or a common cold?

That can be tricky. The flu can cause a fever, headaches, body aches, a cough, a runny nose, and fatigue — all symptoms that can also turn up among people who have a common cold. But the flu tends to cause a fever of 100 degrees or higher as well as headaches and body aches or chills, while the common cold typically does not, says Jeffrey Steinbauer, a professor of family medicine at Baylor College of Medicine. Also, the flu usually doesn’t cause a sore throat, which is a typical symptom among people with the common cold, he says.

Another potential giveaway: The common cold tends to come on gradually during a period of a couple of days, while the flu often lands more abruptly, Steinbauer says. If someone goes from feeling well to feeling crummy in the space of an hour or two, that’s more indicative of flu than of the common cold.

How should you treat symptoms of the flu?

If you’re otherwise healthy, the flu tends to cause unpleasant but mild symptoms. You may feel feverish and stuffed up, but you don’t need urgent medical care. A few days rest on the couch and in bed should clear things up. While you’re taking it easy, over-the-counter drugs can help treat your symptoms; aspirin or acetaminophen will relieve your aches and fever, and a decongestant can turn off your runny nose, Steinbauer says. (Important: the CDC says anyone 18 or younger who has the flu should not take aspirin or salicylate, which can cause rare but serious complications.)

On the other hand, women who are pregnant, those who are 65 or older, kids age five and younger, and people with medical conditions like asthma, cancer, or diabetes are all at high risk for flu-related complications such as pneumonia or bronchitis, according to the CDC. If you’re a member (or a caretaker) of one of these at-risk groups, call a doctor if you think you have the flu.

Your doctor may recommend that you take an “antiviral,” which is a class of prescription-only drugs that knock out flu infections. But you need to start taking antiviral drugs 48 hours or less after your symptoms start, so don’t wait to call your doc.

If you have the flu, you’re most contagious during the first three or four days. But you can still spread the flu for up to seven days. You can also give other people the flu before your symptoms show up.

When do I have to go to the emergency room with flu symptoms?

“Essentially, never,” Morse says. And the CDC agrees: Most people who are infected with the flu do not need to go to the ER. Even if you’re pregnant, sick, or otherwise at elevated risk for flu-related complications, your first call should be to a doctor or clinic.

But there are some flu-related symptoms that justify a trip to the emergency room. If you have trouble breathing, pain that won’t go away in your chest or abdomen, unrelenting dizziness or confusion, severe weakness, or fever or cough that gets worse again after it had noticeably improved, those are all symptoms that — in an adult — may necessitate an ER visit. In kids, the list of symptoms that warrant an ER visit is longer — and includes a bluish face or lips, a fever above 104 degrees, and trouble breathing.

How can you avoid spreading the flu to others?

Flu typically spreads when a sick person’s mucous finds its way into a healthy person’s body. This usually happens when people with the flu sneeze or cough, which sprays droplets of virus-containing mucous into the air where it can be inhaled or otherwise taken up by innocent bystanders. A new study even shows that breathing alone is enough to spread the flu virus. If you can’t stay home while you’re sick, wear a face mask, or, at the very least, cough and sneeze into a tissue or your elbow. These measures can help prevent aerosolized flu germs from sailing into the mouths, ears, or noses of those around you.

Flu can also spread when a sick person touches a railing, doorknob, or other objects that people will be handling. Washing your hands often — especially if you’ve been sneezing, coughing, or wiping your nose — can reduce the number of flu-ridden germs you leave behind.

How long are you contagious?

If you have the flu, you’re most contagious during the first three or four days. But you can still spread the flu for up to seven days. You can also give other people the flu before your symptoms show up. This is why experts say, over and over again, that flu shots are the best way to prevent widespread outbreaks.

Can you get the flu twice in one season?

Unfortunately, yes. “There are often several different flu strains circulating at the same time each year, and you can certainly get one after another,” Morse says. If you’re the type who travels frequently or spends a lot of time in public places like subway stations or airports — places where a lot of people intermingle — you’re more likely to be exposed to different strains of the flu, and so more likely to contract an influenza infection more than once. The one bright spot: If the two strains of flu that you contract are closely related, your symptoms the second time around should be milder, Morse says.

The Vaccine

What’s in the flu vaccine?

Most vaccines contain inactivated or “killed” viral antigens. When the human immune system is exposed to these inactive antigens, it produces protein antibodies just as it would if it encountered live viral proteins. Basically, it develops defenses against the flu without ever having been truly infected.

Nasal spray flu vaccines, as opposed to vaccines given as a shot, contain living but weakened viral antigens. While these nasal spray vaccines tend to work more or less just like a shot, they aren’t recommended for pregnant women, people age 50 and older, and others who may be at elevated risk for flu-related complications. Also, there’s some evidence that these nasal sprays don’t always offer the same protection as a shot. Unless you (or your child) have a real problem with needles, a traditional flu shot is the way to go.

Is it ever too late in the season to get the flu shot?

No. “You would like to be vaccinated before the season starts, but it’s very hard to predict when the season will peak or end, or how many strains will be circulating,” says Pedro Piedra, a flu expert and professor of virology at Baylor College of Medicine.

The CDC says people should consider getting a flu shot as long as flu season persists, which in some years could run all the way into May. On the other hand, getting a flu vaccine too early in the year — say, in July or August — may result in “reduced protection” against the flu strains that emerge later in the season. If you can get your flu shot before the end of October, that’s usually ideal.

Why do some people feel fluish after they get the shot?

In “rare cases,” most of them involving kids, the inactive virus proteins in the flu vaccine can trigger a low-grade fever or other mild flu-like symptoms, such as headaches, Piedra says. These symptoms tend to last for a day or two, which is much shorter than a full-fledged influenza infection.

More commonly, someone who has already contracted the flu or a common cold — but who hasn’t yet begun to feel symptoms — mistakenly blames the vaccine when their symptoms emerge. Piedra says that tens of millions of Americans will develop colds or flu each year, and tens of millions will get the flu vaccine. It’s a statistical certainty that many, many people are going to develop cold or flu symptoms around the time they get the flu shot. But in nearly all these cases, their illness has nothing to do with the vaccine.

What are the chances that you’ll get the flu even if you get a flu shot (and how is that possible in the first place)?

That’s impossible to say for certain because the flu shot’s effectiveness varies from year to year. But experts say that, even in the best years, the flu vaccine is only 50% to 70% effective.

Why so low? There are hundreds of different flu strains, and any number of them may be circulating during a given flu season. The flu vaccine tends to contain antigens drawn from a combination of three or four different strains. But if you’re exposed to other varieties of the flu, the vaccine won’t protect you. “We’re always trying to provide the best guess as to what type of influenza virus will be circulating,” Piedra says. “But if we don’t choose the right antigen components, the efficacy will be reduced.”

While the experts sometimes guess wrong, there’s evidence that symptoms of flu tend to be milder among people who are vaccinated, compared to those who are not, even when the vaccine isn’t optimally matched to that year’s strain. And so getting the vaccine is still a good idea.

What’s a “universal” flu vaccine and when will it be available?

This is a vaccine that provides protection against any and all strains of flu virus. And as of today, it remains a pipe dream. “People are looking into it and working on it, but I would not expect to see it anytime soon,” Piedra says.

Can I get a flu vaccine if I am allergic to eggs?

Yes. According to the CDC, allergic reactions to egg-based vaccines are very, very uncommon, and these vaccines are safe for people with egg allergies. That said, if you’ve ever had a severe allergic reaction to eggs — meaning something more than just hives — the CDC recommends getting your vaccine at a doctor’s office, hospital, or medical clinic, as opposed to in a drug store, supermarket, or some other nonmedical location, as a precaution.

What’s the difference between the cell-based and egg-based vaccine? Is one better than the other?

Up until recently, the FDA required that all flu vaccines be grown in chicken eggs. While egg-grown viruses were long ago established as safe and effective, they come with some drawbacks. “They don’t always produce the best antigens,” Piedra says. Specifically, the dissimilarity between egg cells and mammalian cells can lead to small changes in the influenza antigens that are grown in egg cells. These small changes can cause the human immune systems to develop antibodies that aren’t quite optimized for fighting off the circulating flu viruses.

In 2016, the FDA approved a manufacturing process that uses animal cells instead of egg cells. This new process produces influenza antigens that are more like the ones humans encounter, and should, therefore, induce better protection against the flu, Piedra says. The cell-based vaccine is called Flucelvax, and it’s widely available. But egg-based vaccines are still much more prevalent, and Flucelvax may be hard to find — especially later in the flu season.

Which flu vaccines should I get?

That depends on your age, health status, and other factors. For example, adults age 65 and older are at greater risk for flu-related complications, and so the CDC recommends that they get either a high-dose flu vaccine or an “adjuvanted” flu vaccine, which is a type that includes additional compounds meant to stimulate a strong immune system reaction. Because both of these trigger a stronger immune response than typical vaccines, they’re more likely to produce mild side effects like pain at the injection site or a low-grade fever. But they also offer greater protection against the flu.

Also, Piedra says most vaccines are either “trivalent” or “quadrivalent,” meaning they contain antigens from either three or four types of flu virus, respectively. “Usually vaccines have four strains because that offers better protection,” he explains. If your local clinic or drug store is offering a choice, you’ll want to go with the quadrivalent vaccine, he says.

There are several other types of vaccines. If you’re trying to figure out which one is your best option, talk with your doctor.

Is there anything else besides the vaccine that I can do to kick my immune system into high gear to protect myself from the flu?

If you’re going by word of mouth, a lot of different nutrients or supplements — from vitamin C to probiotics — are supposed to offer protection against the flu.

“But there isn’t much solid evidence” backing any of them, Morse says. A healthy lifestyle — eating right, getting exercise, and plenty of sleep — “is always a good idea,” he adds. But apart from getting a flu shot, there’s nothing in particular that you can do to bolster your body’s defenses against influenza.

Does stress make you more vulnerable to the flu?

Chronic, high levels of psychological stress may mess with the human immune system in ways that leave a person vulnerable to influenza. And research has found that both stress and positive mood seem to change the effectiveness of the flu vaccine.

While stress may make the vaccine slightly less effective, one 2018 study found that being in a good mood on the day of a flu shot leads to hormone shifts that bolster immune system activity, and that consequently improve the flu vaccine’s efficacy.

But at this point, the links between stress and flu risk are murky.

“It seems logical to think that stress could make a person more vulnerable to influenza, but there isn’t enough evidence to be certain,” Morse says.

Should I be concerned about anti-vaxxers when it comes to seasonal flu?

Returning to Morse’s comments about “herd immunity,” it’s a certainty that the influenza vaccine would spare more people from illness and death if more Americans got vaccinated. The lower the vaccination rate, the greater the “disease burden” of influenza, he says. The good news is that, per CDC estimates, more people were vaccinated last year than the year before, which suggests things are moving in the right direction.


Illustrations by Na Kim

This story is part of “The Elemental Guide to This Year’s Flu,” a multi-story special report.

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