Is it okay to get a flu shot if you already have a cold?

Bring on the sweaters, PSLs, and season premieres — fall has officially arrived. On a less cozy note, so has flu season, which means it’s time to get your yearly flu shot, if you haven’t already. Remember, influenza, a contagious respiratory illness, can be fatal; the flu killed over 61,000 people between last October and early May, according to Centers for Disease Control estimates. Besides protecting yourself, getting the flu shot can also help prevent the illness from spreading to babies, older adults, people with compromised immune systems, and other vulnerable groups. But what if you’ve scheduled a flu shot appointment, only to wake up the morning of with a sore throat, stuffy nose, and other telltale cold symptoms? Should you still get a flu shot if you have a cold?

First, though, a crash course on how the flu shot works: The flu shot, or vaccine, contains weakened influenza viruses, the CDC explains. They can’t cause the flu, but they can trigger the immune system to produce proteins called antibodies that help it quickly recognize and mount a defense against actual influenza viruses in case it does encounter them. (Developing these antibodies takes around two weeks.) Each year’s flu shot contains the influenza viruses predicted to be most prevalent in the approaching flu season.

According to the doctors Mic spoke to, whether you should get the flu shot if you have a cold boils down to the severity of your cold symptoms.

If you have a mild cold — say, a low-grade fever of 99 degrees Fahrenheit, with a slight cough and stuffy nose, but nothing that keeps you from work or other day-to-day activities — you’re good to get the flu shot, says Dean Blumberg, chief of pediatric infectious diseases at UC Davis Health. “That’s not much of a burden on the immune system.” In other words, it would still have the bandwidth to make antibodies in response to the shot, keeping you protected from the flu.

On the other hand, if you have a high fever and other serious symptoms that prevent you from going to school or work, your immune system may be especially focused on fighting that cold, Blumberg tells Mic, adding that “it can be so activated that it could have a less coordinated response” to the flu shot. (A fever greater than 100.4 degrees Fahrenheit counts as "high," says John Lynch, associate professor of medicine at the University of Washington School of Medicine.) As a result, you might be less protected from the flu later on.

Plus, the flu shot can cause minor side effects, such as a low-grade fever and fatigue, which could make you feel even worse if you already have a severe cold, Lynch says. In extremely rare cases, it can result in a severe allergic reaction.

Blumberg adds that these side effects can also make it hard for your doctor to tease apart what’s going on. If your symptoms worsen, is it because of your horrible cold just running its course, or a side effect of the vaccine?

If you have a cold with a high fever, or are visibly unwell, many clinicians would ask you to wait until your fever subsides and you start to feel better before getting the flu shot, says Margaret Khoury, a pediatric infectious disease physician and chief of pediatrics at Kaiser Permanente Baldwin Park Medical Center in California. Usually, that amounts to only a few days, the typical lifespan of a cold. “We just want people to get [the flu shot] as early as possible,” Lynch says — all the more reason to get it now, before you come down with a cold, or to drag yourself to your flu shot appointment if you have only mild cold symptoms, especially if it took some effort to schedule.

It’s important to note that if you already had a bad cold and feel worse after getting the flu shot, it’s not because the shot made your cold worse, Lynch says. Rather, it's added a layer of side effects to your cold symptoms.

The take-home message: In most cases, cold-havers can rest assured. “It is generally safe to get a flu shot if you have a cold,” Lynch says. “The flu vaccine cannot give you the flu, nor can it give you any other infection. It can’t make you get a cold, and it can’t make your cold worse.”