Most cases of COVID-19 are mild or even asymptomatic. Similarly, most patients seem to make a full recovery. But some patients seem to suffer significant lung damage, according to doctors at the Hong Kong Hospital Authority (HKHA).

“They gasp if they walk a bit more quickly,” Owen Tsang, medical director of the HKHA’s Infectious Disease Center, said in a press conference. “Some patients might have around a drop of 20 to 30 percent in lung function.”

There are still many things we don’t know about the COVID-19 outbreak — and there is even less that we know about how recovery from the disease takes place.

Now, in a new preliminary study, doctors from the Hong Kong Hospital Authority published a case study of the first wave of patients who were discharged from the hospital and had fully recovered from COVID-19.

Out of 12 people hospitalized following a coronavirus infection, 2 or 3 saw changes in their lung capacity — a significant decrease.

It’s still a very small sample size and it’s too early to say what are the chances that COVID-19 might cause long-term lung damage, but it’s yet another aspect that should be taken into consideration as we progress into the outbreak.

“Among those who are infected, most will recover,” the World Health Organisation’s director-general, Tedros Adhanom Ghebreyesus, said on Monday. 

This study is also a reminder that we don’t fully understand the effects of the disease. We do know that most cases (over 80%) are mild, and the elderly and immunocompromised are at the highest risk (although younger people can also be severely affected). But we don’t know much about the long-term consequences of the disease, and even among recovered cases, there might be lingering effects.

Discharged patients could do cardiovascular exercise such as swimming to help the lungs recover gradually.