Slightly different is the psak of the dual Chief Rabbis of Bnei Braq, Rabbis Shevach Rosenblatt and Chaim Yitzchak Landau.

the Chief Rabbis of Bnei BRaq published a letter with their guidance on how to act on Tisha B'Av this year.

the main points and differences from Rabbi Lau are:

Everyone should be extra cautions, wear masks, wash hands, keep distance, etc

On Tisha B'Av itself one can use alcogel on his hands as that is not washing but is disinfecting, and especially in light of the danger it is allowed. They do not say anything about washing hands with soap on Tisha B'Av - presumably they are of the opinion one should not, except perhaps in a situation of clear danger, but that is just my assumption based on its absence.

Tisha B'Av is a more stringent fast than many of the others, so, upon consultation with God Fearing doctors:

Anyone healthy and over bar/bat mitzva, even if in quarantine, must fast

Anyone who tested positive for CoronaVirus but has no symptoms at all, must fast

Anyone who has symptoms, even light symptoms, must NOT fast

Anyone fasting and over the course of the day develops symptoms, such as fever, coughing, achy muscles, loss of taste and smell, etc, must break the fast and eat (how is he going to know if he lost the sense of taste?)

Anyone who has suffered from CoronaVirus over the past two months in a serious way, such as with pneumonia or shortness of breath, should NOT fast even if right now he has no symptoms

Stay healthy and may our mourning turn to joy