A Japanese research institute says one in four people continued to experience suspected coronavirus aftereffects even 18 months after infection.

The National Center for Global Health and Medicine interviewed 502 people in their 20s to 70s who were infected between February 2020 and November 2021.

The center found that 32.3 percent complained of possible aftereffects six months after they got the virus.

The figure fell to 30.5 percent one year after infection, and to 25.8 percent another six months later.

As for symptoms one year after infection, 11.7 percent complained of memory loss, 11.4 percent cited lack of concentration and 10.3 percent had problems with their sense of smell.

Brain fog affected 9.1 percent of the former patients, while 7.5 percent suffered psychological distress.

The center says women tend to be affected by an abnormal sense of smell, hair loss and lack of concentration.

Those who had moderate to severe COVID-19 symptoms often experience shortness of breath, cough and lethargy.

Doctor Morioka Shinichiro says aftereffects are said to be less frequent among people infected with the Omicron variants, but they should not be underestimated because case counts are so high.

He urges people to keep taking anti-infection measures, warning that aftereffects could last long even if symptoms of infection are mild.