Editor's note: This popular story from the Daily Briefing's archives was republished on June 21, 2023.

According to a new  analysis  published in JAMA Internal Medicine, six factors could significantly increase your risk of developing long COVID — and one factor could protect you.

The biggest risk factors for long COVID

For the analysis, a team of researchers from Britain examined data from 41 studies published between the beginning of the pandemic and Dec. 5, 2022. A total of 860,783 patients were included in the study.

Using the data, researchers examined patients' risk of developing symptoms of long COVID, including fatigue, shortness of breath, brain fog, and a loss of taste and smell, more than three months after their initial infection.

Overall, the researchers found that six risk factors were associated with an increased risk of developing long COVID:

1. Being a woman

According to the analysis, women were 1.5 times more likely to develop long COVID than men. Female hormone levels may prolong inflammation after an initial infection or higher levels of certain antibodies in women may cause symptoms to linger.

2. Being older than 40

Age was a significant risk factor for long COVID, and patients with the condition were roughly 20% more likely to be older than 40.

Although the analysis found that people ages 70 and older had the same risk of long COVID as those ages 40 to 69, the researchers said this might be due to the fact that patients older than 70 were more likely to die from their initial COVID-19 infection.

3. Being obese

Patients who were obese were around 1.15 times more likely to develop long COVID than those who were not. According to the researchers, obesity can involve a metabolic inflammatory process that could prolong COVID-19 symptoms.

4. Being a smoker

Although smoking was identified as a risk factor for long COVID, the researchers noted that it was unclear whether this was due to the smoking itself or health conditions associated with smoking.

5. Having other medical conditions

Several different medical issues were examined in the study, and the researchers found that patients with immunosuppressive conditions had the highest risk of long COVID, followed by those with chronic obstructive pulmonary disease, ischemic heart disease, or asthma.

There was also an increased risk of long COVID for patients with anxiety, depression, chronic kidney disease, or diabetes.

6. Being hospitalized during their initial COVID-19 infection

According to the analysis, patients who were hospitalized or admitted to the ICU during the acute phase of their COVID-19 infection had a significantly higher risk of developing long COVID. Patients in either of these two groups were over twice as likely to develop long COVID than those who were not hospitalized or admitted to the ICU.

"Patients with previous critical illness represent a high-risk population, and their follow-up should reflect intensive plans for prevention, rehabilitation and treatment of the ongoing debilitating symptoms," the researchers wrote.

However, they also noted that since most people who have had COVID-19 did not need to be hospitalized, a greater number of long COVID patients likely had milder initial infections.

What can lower your risk of long COVID?

Similar to other studies, the researchers found that vaccination against COVID-19 could reduce the risk of developing of long COVID. Patients who had two doses of a COVID-19 vaccine were 40% less likely to develop long COVID than those who were unvaccinated.

And another recent report from the U.K. Office of National Statistics found that two doses of a COVID-19 vaccine was associated with a 42% lower risk of long COVID.

According to the researchers, emerging evidence suggested that vaccination could reduce the risk of long-term COVID-19 symptoms "even in individuals with other risk factors, such as older age or high B.M.I."

Overall, patients with long COVID "may experience long-lasting adverse effects requiring long-lasting support" and "[f]ollow-up outpatient services may be needed to manage this condition and to better understand the possible association between symptoms and residual organ impairment," the researchers wrote.

However, the researchers added that since "health care systems worldwide have been substantially burdened by the COVID-19 pandemic, routine follow-up may not be possible to all those living with" long COVID. (Belluck, New York Times, 3/23; Prater, Fortune, 3/23; Tsampasian et al., JAMA Internal Medicine, 3/23)