3 mRNA jabs enough to keep healthy people well-protected, even for new Covid-19 variants: Expert

The vaccines play a big part in preventing severe illness and death as they would have primed other functions of our immune system. ST PHOTO: NG SOR LUAN

SINGAPORE - People in Singapore who have received their mRNA booster shots are well protected against severe Covid-19, and this applies even when it involves new virus variants like the Kraken, said Professor Ooi Eng Eong, an expert in emerging infectious diseases at Duke-NUS Medical School.

This is what is important now that Covid-19 is endemic – that people do not get severely ill or die from the disease, he said.

Neutralising antibodies, which protect against infection and which still seem to be the focus of much of the world, are no longer as important in an endemic situation, particularly as they wane over a few months.

Vaccines play a big part in preventing severe illness and death, as they would have primed other functions of the immune system. Prof Ooi said: “These are also potent in limiting the spread of infection in our bodies.”

Likening the immune system to the Singapore Armed Forces defending the country against invasion, he said: “Neutralising antibodies can perhaps be thought of as the first line of defence. If the invading forces slip through that net, then the other arms of the army, for example, artillery, armoured vehicles and special forces, can be deployed and reservists mobilised; that’s on top of the air force and navy.” 

The vaccine similarly primes all of the body’s different defensive mechanisms, of which neutralising antibodies are just one. Because they are the easiest to measure, they are the one most commonly referred to.

He said this was shown by phase three clinical trials of the mRNA vaccines. Against the placebo arm, the number of Covid-19 cases in those vaccinated started to tail off 12 days after the jab, although it takes 21 days following vaccination for the neutralising antibodies to appear.

Prof Ooi said: “Whatever that protects must happen at around day seven to day 10. The data shows it is sufficient to protect against symptomatic infection. At that time, there were antibodies, but there were absolutely no neutralising antibodies.”

He added that although only 2.2 million people here are officially listed as having been infected, experts estimate that roughly 90 per cent of the population have had a brush with the Sars-CoV-2 virus.

If they have been vaccinated, they would have hybrid protection, which provides even better defence against severe sickness than either vaccination or infection alone.

He said: “There is sufficient data now to be confident that in those who are not immunocompromised by chronic diseases or medication, the resultant hybrid immunity protects robustly against subsequent symptomatic infection, and especially severe Covid-19.” 

So, for healthy adults, he said, one booster shot – that is, three mRNA jabs, or four if it is Sinovac – should provide enough protection against severe Covid-19 disease, even with new variants. This is what Singapore refers to as minimum protection – which 83 per cent of eligible people here now have.

While new emerging variants might be somewhat different, they all share over 90 per cent of the viral proteins, which the immune system would recognise. It might take slightly longer to do so, but in a normal healthy person, the body will mount a response in time to prevent severe disease.

“For them, the benefit of an additional jab is minuscule, because they already have very good protection. Then the risk of even rare events like myocarditis becomes very real,” he said, adding that in medicine, you only do something, including vaccination, when benefits clearly outweigh risks.

However, people who are at higher risk might need to take boosters regularly because their natural immunity is lower. This includes the elderly, people with poorly controlled diabetes, cancer patients on certain medication, and transplant patients. For them, protection against infection might be important.

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