Getting people who have recovered from Covid-19 to take at least one vaccine dose is a reasonable precaution, say infectious disease experts, given that not much is known about the strength of immunity from natural infection and how long this protection lasts.

What scientists know for certain is that a small number of people have been infected twice, with studies showing immunity wanes in recovered patients over time.

"In a way, we are regarding the actual infection as the first dose of a vaccine, therefore only one jab is needed," explained Professor Dale Fisher, a senior infectious disease consultant at the National University Hospital.

Last month, the Ministry of Health said people who have recovered from Covid-19 will get priority for a single dose of the vaccine, as long as it has been more than six months since their infection.

This is because they are likely to still have a strong immune response in the first six months after being infected.

In an update last Thursday, the ministry added that 51,541 people who had recovered from Covid-19 have received at least one dose, completing the full vaccine regimen.

When a previously infected person gets a single shot of an mRNA vaccine - that is, either the Pfizer-BioNTech or Moderna jab - their bodies show an increase in neutralising antibodies that help fight Covid-19.

This spike is equivalent to, or even higher than, the antibody levels of uninfected people who have received two doses of the vaccine, said Associate Professor Hsu Li Yang, an infectious disease specialist at the National University of Singapore's Saw Swee Hock School of Public Health.

However, a second dose of the vaccine for those who have recovered does not result in even more protection, he observed.

He also noted that people who have recovered from Covid-19 have varying levels of such antibodies, depending on the severity of their infections.

Globally, public health experts and national health agencies are recommending that recovered patients still get jabbed, although their advice is not uniform.

In the United States, recovered patients are encouraged to get two doses of the vaccine as soon as they can, while in parts of Europe such as France and Italy, such patients are recommended a single dose.

One reason Singapore requires Covid-19 patients to wait at least six months before getting vaccinated is to avoid immune exhaustion - that is, making sure there are "fresh immune cells" to respond to the vaccine, said Professor Paul Tambyah, who is president of the Asia Pacific Society of Clinical Microbiology and Infection.

He added that another reason is to avoid the theoretical risk of immune enhancement, when vaccination triggers an overreaction from an "already primed system".

Added Prof Tambyah: "Many of my recovered patients have gone for their one-dose vaccination recently without any issues."