Fully vaccinated people protected from Covid-19 but can pass disease to others

Serology tests detect antibodies in the patient's blood sample- antibodies being the soldiers of the immune system which help the body defeat invaders.
Serology tests detect antibodies in the patient's blood sample- antibodies being the soldiers of the immune system which help the body defeat invaders.PHOTO: LIANHE ZAOBAO

SINGAPORE - People who are fully vaccinated against Covid-19 generally do not get seriously ill even if they get infected with the virus. However, they could still transmit it to others.

Associate Professor Raymond Lin, director of the National Centre for Infectious Diseases (NCID) National Public Health Laboratory, told The Straits Times that this was the centre's observation from studying the serology test results of those diagnosed with Covid-19.

Serology tests detect antibodies in a patient's blood sample - antibodies being the soldiers of the immune system which help the body defeat the invader.

Thus, these tests can detect past infections or whether a person has been vaccinated.

This ability differentiates serology tests from other diagnostic tests that flag current infections.

The latter include polymerase chain reaction tests, which look for snippets of the virus' genetic code, and antigen rapid tests that pick up the presence of viral proteins in a sample.

In some cases, serology tests help scientists and clinicians determine whether the antibodies were produced by natural infection or through vaccination.

Knowing this can help public health officials identify "vaccine breakthrough" cases - or those who get Covid-19 even after being fully vaccinated against it.

Said Prof Lin: "Defining vaccine breakthroughs will help us study how often (such cases) occur, whether they can transmit infection, whether they develop severe disease and whether new variants infect vaccinated subjects more easily."

He added: "So far, we observe that some infected but vaccinated subjects may transmit the virus to others. Generally, vaccinated subjects do not develop severe disease when they are infected."

Similar to how soldiers, sailors and airmen are mobilised in times of war, the immune system also summons different types of antibodies to get rid of the invading pathogen.

Vaccinations mimic this fight.

The Moderna or Pfizer-BioNTech/Comirnaty messenger ribonucleic acid (mRNA) vaccines - both currently used in the national vaccination programme - work by delivering instructions for the human cells to begin producing the viral spike (S) protein. This S protein studs the surface of the virus and is what the pathogen uses to begin infecting cells.

When S protein fragments are produced, the body recognises them as invaders and summons antibodies that target the S protein. "Therefore, the serology tests should detect anti-S antibodies as evidence of vaccination," said Prof Lin.

However, when natural infection occurs, the body is exposed to the whole virus and has to summon antibodies to deal with other viral proteins and not just the S protein.

If someone has already been infected by the coronavirus, serology tests will pick up the presence of antibodies against the virus' nucleocapsid (N) protein - which is found wrapped around the viral genetic code.

In essence, those who receive the mRNA shots will show the presence of antibodies against the S but not the N protein, unless they have been infected before.

However, vaccines such as Sinovac's whole virus one will prompt the body to produce antibodies against the S and N proteins.

Whole virus vaccines work by using a weakened or deactivated form of the virus to trigger protective immunity against it.

For instance, Sinovac's vaccine comprises a whole coronavirus that is killed chemically before being injected into the body. The virus can no longer replicate, but its proteins are intact.

In this way, the immune system is exposed to the coronavirus' various proteins.

Prof Lin said it may not be possible to tell in such cases whether antibodies are produced as a result of vaccination or natural infection. "However, we have not had enough of such cases to study the expected antibody response," he added.

Sinovac's Covid-19 vaccine was recently made available under the Special Access Route, although it does not come under the national vaccination programme.

The first of the Sinovac shots were administered at some private clinics on June 18.