Covid-19 and the Flu

Long before the coronavirus that causes Covid-19 emerged, annual outbreaks of influenza posed a serious threat. During the pre-pandemic 2018–19 flu season—considered to have had a peak activity of moderate severity for the flu—the United States recorded 35.5 million cases of influenza and more than 34,000 flu-associated deaths.

In contrast, the 2020-21 flu season saw a dramatic drop in cases. In fact, that season had the fewest number of hospitalizations for influenza of any other year on record. Experts attributed the drop to protective measures taken to stop Covid from spreading, including mask-wearing and social distancing, which also protect against the flu. Increased rates of flu vaccination helped as well.

Will the 2021–22 season see the same drop in hospitalizations? Experts aren’t sure, but some are concerned that we’ve lost some infection-induced immunity to the flu as a result of not being exposed to the virus last flu season. Combine that lack of exposure with relaxed Covid restrictions in many places around the country, and we could be in for a rough season—and a potential “twindemic” of influenza and Covid. As of December, we’ve seen an uptick in flu cases, especially among younger adults.

The truth is, no one knows for sure what will happen this flu season. But we do know that the best way to prevent a collision course between the two viruses is to get vaccinated against both Covid and the flu.

In the meantime, here are the latest answers to some frequently asked questions about the similarities and differences between Covid and the flu.

Q. Do the viruses that cause Covid and influenza spread in the same way?

Both viruses spread via droplets expelled by an infected person who coughs, sneezes, or simply talks within about six feet of you even though the droplets settle to the ground within seconds. What’s more, both viruses, particularly the one that causes Covid, also spread through tiny aerosol particles that travel in the air much farther than six feet and can remain aloft for hours. This latter mode of aerosol transmission occurs in enclosed spaces and rarely outdoors. While inhaling the airborne virus is the most common way people become infected, it’s possible you can also contract both influenza and Covid by touching a contaminated surface or an infected person (such as when shaking hands) and then touching your nose, mouth, or eyes. Covid spreads more easily from person to person than influenza does. There’s also evidence that Covid may be more prone to spread via “superspreader” events involving large gatherings of people.

Q. What is the timeline for Covid and flu symptoms, and for how long is a person contagious?

Covid symptoms most commonly begin around three days after infection with the Omicron variant; symptoms typically occur four to five days after infection with Delta. But symptoms may appear anywhere from two to 14 days. People with Covid generally can spread the virus for about one to two days before symptoms begin, and they remain most contagious for two to three days after, according to the CDC. People who have no symptoms are typically contagious for up to five days after becoming infected. However, the length of time a person with Covid is contagious is still being studied.

Flu symptoms typically appear one to four days after infection, and the flu can be contagious beginning a day before symptoms appear. Adults remain contagious for up to five to seven days after symptoms begin, children a bit longer.

Q. Are the symptoms of the flu and Covid the same?

For the most part, yes—and this is one reason why this flu season could be so challenging for doctors. In addition, people vulnerable to Covid are also at increased risk for serious complications from influenza. And since these illnesses share many signs and symptoms, doctors will likely not be able to distinguish between the two without testing.

Both viruses range from having no symptoms to having severe, life-threatening symptoms. Common symptoms include:

• Cough
• Fever and chills
• Aches and pains
• Shortness of breath or difficulty breathing
• Sore throat
• Fatigue
• Headache
• Runny or stuffy nose
• Nausea, vomiting, and diarrhea (more common in children)

A change or loss of sense of taste or smell sometimes occurs with Covid and less commonly with the flu. That symptom might help tip off doctors to the diagnosis.

Q. How can doctors tell for sure if you have Covid or influenza?

The only way to know for certain is with a diagnostic test. The Food and Drug Administration (FDA) has authorized several tests that can distinguish between both viruses, with one sample having the ability to check for both flu and Covid. A rapid flu diagnosis is important since antiviral medications may reduce illness length and symptom severity, but only if they’re given within 48 hours of symptom onset. The FDA recently granted emergency use authorization to two oral antiviral agents for treating Covid; they are most effective when given as soon as possible—within the first three to five days of illness—but are not readily available yet. Monoclonal antibodies may also help in treating Covid.

Q. Can you be infected with both viruses at the same time?

It’s possible—but not very common—to be infected with Covid and other respiratory viruses at the same time, including influenza, common cold viruses, and respiratory syncytial virus (a respiratory infection that causes cold-like symptoms, coughing, wheezing, 
and sometimes pneumonia). For instance, an analysis of 23 studies, published in Frontiers in Medicine in June 2021, reported that Covid and flu co-infections occurred in a small percentage of patients. Even so, the researchers wrote that the possibility of co-infection in people at high risk and older adults should not be ignored. You’ll lower your risk of a dual infection by getting both the Covid and flu vaccinations. Plus, the Centers for Disease Control and Prevention (CDC) says you can even get both shots at the same time, in different arms.

Q. Will the flu vaccine also help prevent Covid?

No. The flu shot protects against influenza only, not other respiratory viruses. But getting your shot this flu season is particularly important. Given the rising number of Covid cases due to the Omicron variant, coupled with healthcare staff shortages, many hospitals around the country are again struggling to meet patient needs and may soon become overwhelmed—so anything and everything we can do to stay healthy will help ease the healthcare burden. And there’s solid evidence that the flu vaccine prevents millions of illnesses, doctors’ visits, and hospitalizations.

During the 2017–18 flu season, flu vaccinations prevented an estimated 6.2 million influenza illnesses, 3.2 million influenza-
associated medical visits, 91,000 influenza-associated hospitalizations, and 5,700 influenza-associated deaths, according to the CDC. Getting vaccinated reduces the risk of having to seek medical care for the flu by 40 to 60 percent.

According to the CDC, flu shots reduce the risk of flu-associated hospitalizations among older adults by an average of about 40 percent. Getting a flu shot lowers the risk of older patients being admitted to an intensive care unit by as much as 82 percent.

Plus, preliminary evidence from a study in PLOS ONE in August 2021 suggests that even though the flu vaccine won’t prevent Covid, it may help prevent severe effects of Covid if you become infected, possibly by boosting the innate immune system. The study found lower rates of ICU admissions, strokes, sepsis, and other adverse outcomes after a Covid diagnosis in people who had received the flu vaccine compared to those who hadn’t.

Q. Is there a special flu shot for older people?

Yes. Because immune responsiveness declines with age, people 65 and older may not generate sufficient protection from the standard flu shot. For that reason, researchers have developed a more potent shot that gives the immune system an added boost. In 2021, the FDA approved two vaccines for people 65 and older that should offer wider protection against circulating strains of the flu. They cover the same four strains of influenza as the vaccines for younger people do.

Q. When should I get my flu shot?

October, just before influenza season begins, is ideal. However, if you aren’t vaccinated by then, you can still get your shot anytime during the flu season when viruses are still circulating. It takes about two weeks after the jab to develop protective immunity, and that immunity tends to wane, so you can only count on about six months of protection. Flu season typically peaks between December and February, but viruses can be active as late as May.

Unfortunately, too many people don’t take advantage of this preventive shot. (A flu vaccine in the form of a nasal spray is also approved for most people ages 2 to 49.) The CDC estimates that 55 percent of Americans got inoculated against the flu during the 2020-21 flu season.