The Straits Times article "Some private clinics begin administering Sinovac vaccine" (June 19) reported that the patients coming in for the Sinovac vaccine are mostly from the Pioneer Generation as well as those who cannot take the Pfizer-BioNTech and Moderna vaccines for medical reasons.

As a physician and rheumatologist, I struggle to understand the medical reasons which would allow people to opt for the Sinovac vaccine yet not the mRNA vaccines.

With the Ministry of Health's expanded inclusions, pretty much everyone, including those with severe allergies (except allergy to the vaccine constituents themselves), is now eligible for the mRNA vaccines.

For people unable to get the vaccine due to a severely suppressed immune system, the concern is regarding the immunogenicity (or effectiveness) of the vaccine and not its safety. Therefore, it applies to any vaccine, not just the mRNA vaccines.

Vaccines work by triggering the body's own immune system to produce antibodies against the virus. If the body's immune system is weak, it may not produce antibodies at all, or produce fewer antibodies. This poor immune response can be circumvented by timing the vaccination to be given when the immune system recovers - for example, at least three months after an organ transplant.

For most people on treatment for autoimmune or rheumatic diseases, this is thankfully not a significant concern, and vaccination is effective without the need for timing or interruption of treatment.

Elderly people and those on immunosuppression are at high risk of severe Covid-19 complications; this makes them a vulnerable group whose protection should be prioritised.

In Singapore, we are privileged to have the mRNA vaccines, the "Rolls-Royce" of vaccines, available to us free of charge.

I encourage all Singapore residents to exercise this privilege and get their shot without delay.

Manjari Lahiri (Dr)