Getting mild infection after jab is good news

Getting infected but not sick is the best thing that could happen to someone who has been vaccinated, as this could protect him against future coronavirus variants, said Professor Ooi Eng Eong of the Duke-NUS Medical School.

"We would be a lot more resilient if we have a vaccination and an asymptomatic infection," he said on Thursday during The Straits Times panel discussion titled Endemic Covid-19: When And How?

Current vaccines teach the body to recognise the spike proteins on the surface of the virus, but that could change as the virus mutates.

Getting an infection teaches the body to recognise all parts of the virus, including the core that cannot change, and helps it mount a more effective immune response against any mutation, he said.

He agreed with Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases, that the higher the antibody level, the better the protection.

However, Prof Ooi pointed out that antibodies do not remain high permanently. These antibodies are generated following vaccination.

"The immune system does not sustain antibodies at a high level when it doesn't need it," he said, but the memory of how to make those antibodies remains.

"When you see the virus again, it will recall this memory and it will mount an immune response. But that memory recall takes a few days. And so it becomes a race between the immune system and Delta."

So far, data shows that more people infected by the Delta variant get sick compared with earlier variants of the virus.

Prof Ooi said this is because of the shorter incubation period - the point from infection to disease - of three days for Delta against five days for the original strain. The immune system, however, takes about four or five days to mount a response.

This is why symptoms may appear before the body is able to properly fight the infection. However, once the body's defence mechanism kicks in, it is often able to prevent serious illness.

Antibodies prevent the virus from entering our cells to replicate. But once the virus has entered the cells, the antibodies are powerless.

However, the body also has defensive T-cells that can be trained to kill infected cells - and the millions of viruses replicating within those cells. The current vaccines train the T-cells to recognise the spike proteins, and if these change significantly as the virus mutates, the T-cells become less effective in fighting coronavirus infection.

A mild infection, on the other hand, would teach the T-cells to recognise the entire virus, including the unchangeable core.