The author of the study answered my question as follows:

"Dear Mehdi,

Thanks for your question.

It is unusual to see this pattern but it makes sense in this case.

The reason is that HIV group are younger than the non-HIV patients. Note that in Table 1 only 4.2% are aged over 70, compared with 17.8% aged over 70 in the non-HIV group. Younger age is associated with lower risk of COVID-19 death.

So when you compare HIV and non-HIV patients, without adjusting for age, the younger age of the HIV patients (which means lower risk) effectively seems to cancel out the higher risk associated with HIV itself, leaving an HR close to 1.

But when we adjust for age, we “remove” age from the HIV vs non-HIV comparison. We are now effectively saying “what is the association between HIV and COVID-19 death, among people of a similar age…?”. The true (positive) association between HIV and COVID-19 death thus emerges.

I hope that helps.

best wishes,

Krishnan"

Krishnan is right - while I think that any correlation with merely the diagnosis is not so relevant (since fortunately with early initiation of ART people living with HIV surely have a near normal immune system etc) - statistically adjustment for different factors in a cohort can go either way - positive and negative for respective factors. Do not think this study is to helpful to understand what is going on - also outdated (2021 paper)

authors write...:

"For similar reasons, we were unable to include data on antiretroviral therapy use, viral suppression, CD4 count, or previous AIDS-defining illnesses likely to be captured only in specialist care, so it was not possible to stratify results or quantify how much of the increased risk was driven by the minority of individuals with poorly controlled HIV. "

Julian Schulze zur Wiesch Thanks. It was published in the Lancet. There is no guarantee that similar limitations would not appear in other studies.