Expert explains why combination of jab, infection can be so powerful

People were intrigued when an infectious diseases expert said last week that the best protection against Covid-19 comes from getting a mild infection after being vaccinated.

Getting both a jab and a mild infection provides better protection against current as well as future variants - compared with just vaccination or infection alone.

Professor Ooi Eng Eong of the Duke-NUS Medical School said the current surge in cases in Singapore may help the population gain more robust immunity than from vaccination alone.

His remarks at a panel discussion organised by The Straits Times that was titled Endemic Covid-19: When And How? sparked the debate over whether an inactivated-virus vaccine like Sinovac works just as well as getting infected because the entire killed virus is used.

"No," said Prof Ooi, who explained that getting infected by a live virus is very different from getting vaccinated with an inactivated virus, as the killed virus lacks the part that allows it to replicate itself once it gets inside human cells.

While such vaccines produce antibodies, they do not trigger killer T-cells.

The body has two main ways to fight an infection.

Antibodies prevent the virus from entering the cells, which it needs to do to replicate itself. This is the first line of defence.

Should this fail, and the virus manages to enter our cells, the second line of defence kicks in.

This involves the killer T-cells that destroy cells which have been invaded by the virus, so none of the replicated virus can escape to infect more cells.

Messenger RNA vaccines (such as the Pfizer-BioNTech/Comirnaty and Moderna shots) generate both antibodies and killer T-cells. But these killer T-cells are based on the code from the original virus used to create the vaccine.

Should the virus change too much, it may affect the response - a possible reason for the lower protection the vaccines confer against the current Delta variant.

When someone gets infected, the body recognises the invader and develops killer T-cells in response.

Killer T-cells generated by an infection will remember the entire virus, including the core machinery that allows the virus to replicate - this part remains unchanged even if other parts of the virus mutate in the future. So, the T-cells will be able to identify mutations and mount a defence.

Vaccines also trigger the development of antibodies. High levels of antibodies can stop an infection. However, the level of antibodies generated after vaccination wanes over time.

This happens with all vaccines. Experts say that if antibodies against all infections remain high all the time, our blood would be sludge.

However, the immune system develops a memory of how to create antibodies, so when an infection occurs, our immune system will generate such antibodies.

But it takes a few days for this to happen. Vaccines are losing the battle against infection owing to the highly transmissible Delta variant, which replicates much faster than antibodies can be generated in the first few days following infection.

However, the benefits of vaccines usually kick in before most people get severely ill.

Getting infected alone does not give the same level of protec-tion as a combination of vaccine and infection, said Prof Ooi, as an infection can turn off some of the body's signals. A vaccine offsets this. Together, they provide both breadth and depth against future variants.

As to why Singapore does not then let the virus do its job and infect people, now that more than 82 per cent of the population has been fully vaccinated, Prof Ooi said that while the theory makes sense, it is extremely difficult to execute.

"How do you keep it tight enough in a way that there is going to be benefit without all the bad things that come with an overwhelmed healthcare system? That part is very difficult."

There is also no guarantee that someone who gets infected suffers only a mild illness, rather than a severe one that could end in death.

Prof Ooi added: "What is happening now is exactly what is needed to become more resilient in safeguarding against new variants.

"That the Delta variant is now spreading in Singapore, but not to the level that results in a large number of severe cases, will serve to produce more robust immunity in our population. I think we should stay the course and not go back to heightened alert or a circuit breaker."