Long COVID is a post-viral illness that occurs after infection with COVID. Also known as PASC, or post-acute sequelae of COVID, it’s  typically defined as new symptoms that develop after infection and persist for at least four weeks—often for months, and sometimes for years.

Similar post-viral illnesses can occur with other viruses, too, like the flu, herpes, Lyme disease—and even Ebola and SARS. Post-viral illnesses often have a chronic fatigue syndrome-like presentation, with symptoms like fatigue, brain fog, and post-exertional malaise, in which symptoms get worse after mental or physical activity.

When looking at the percentage of those infected who develop post-viral illness, as well as the severity of symptoms, post-viral illness from COVID generally isn’t more of a problem than that from flu—in the Omicron era, at least. That’s according to new findings presented this week at the European Congress of Clinical Microbiology & Infectious Diseases in Copenhagen. 

The research took place among residents of Queensland, Australia, where more than 90% of the population had been vaccinated before Omicron burst onto the global scene in early 2022.

For 12 weeks, researchers followed nearly 2,200 adults who had been diagnosed with lab-confirmed COVID, and nearly 1,000 adults who had been diagnosed with lab-confirmed flu. Of those who experienced Omicron, a fifth (21%) had ongoing symptoms at 12 weeks, and 4% reported symptoms that had a moderate or severe impact on daily living.

When it came to those who had experienced the flu, the numbers were almost identical, with 23% reporting ongoing symptoms at 12 weeks, and 4% reporting moderate or severe impacts on daily living.

But there’s a reason why long COVID is having an outsized impact on the health care system, researchers from Queensland Health said in a news release: the sheer volume of COVID infections.

There have been more than 11 million lab-confirmed COVID infections over Australia in the last three years, according to the World Health Organization—a number that is likely a massive undercount.

By comparison, Australia only recorded only a little more than 225,000 lab-confirmed cases of flu last year.

“In our highly vaccinated population, the public health impact of long COVID does not appear to result from any unique property of SARS-CoV-2,” Dr. John Gerrard, Queensland’s chief health officer, said in the release. “Rather, the impact results from the sheer number of people infected over a short period of time.”

Long COVID risk factors

Interestingly, the study noted that those who were members of the following groups were less likely to report moderate or severe functional limitations on their daily lives after Omicron or flu:

• older individuals
• members of indigenous populations

Around 5% to 10% of Australians who’ve had COVID report symptoms lasting for longer than three months. People who are part of the following groups seem to be at greater risk, according to the country’s federal health agency:

• female
• middle age
• those who’ve had severe COVID

In general, people with the following health conditions are at a greater risk of developing long COVID, according to a March journal article in Health Affairs: 

• hypertension
• chronic lung disease
• obesity
• diabetes
• depression

How to prevent long COVID

The best way to prevent long COVID continues to be to prevent COVID infection, by doing the following, according to the U.S. Centers for Disease Control and Prevention:

• masking
• social distancing
• frequent, quality hand-washing
• staying away from those with known illness

It’s widely thought that COVID vaccination could reduce the risk of developing long COVID. Just a single shot could reduce your risk by 35%, according to Harvard Health.

When it comes to reducing long COVID risk, “vaccinating goes a very long way,” Dr. Luis Ostrosky—chief of infectious diseases and epidemiology at UTHealth Houston and Memorial Hermann in Texas, and the infectious diseases director for UTHealth Houston’s COVID-19 Center of Excellence—tells Fortune.

How to prevent post-viral illness from the flu

If you’re looking to prevent potential post-viral illness from the flu, the same recommendations apply, Ostrosky says: “The premise is that the best way not to get long COVID or post-flu issues is to not experience the diseases themselves.”

“COVID was a real eye-opener for all of us as to the power of masking and hand hygiene,” he adds. “It’s no longer automatic to get sick in the winter. You can very likely avoid it.”

A COVID vaccine won’t help you when it comes to the flu. But a flu vaccine could reduce your risk of developing post-viral complications from the flu, he advises.