Existing vaccines versus new variants

Subject : Science and technology

Section: Biotechnology

Concept :

As COVID19 cases began rising yet again in India in March, many wondered whether the existing vaccines, based on the SARSCoV2 virus that was first reported in China, would still be effective against newer versions of the same virus.

Two pronged response

Vaccines generate a two pronged immune response. The first is the production of antibodies by B cells, a type of white blood cells.
Antibodies directly attack and destroy viruses. The second is the Tcell response. T cells are another type of white blood cells.
They have many roles, of which one is to patrol the body and destroy virus infected cells. Both these arms also give rise to specialised memory cells, which are stored away for future needs.
These two commodities are ‘freshly made’ by our body following an encounter with the antigen, introduced during vaccination.
Soon after vaccination, our antibody levels go up. This provides an early window of protection from infection.
However, the levels of ‘freshly made’ antibodies start dropping within three months or so, and eventually plateau to a low baseline. This low level is not enough to prevent infection later.

Why do the levels drop?

It is natural for the body to scale down the production of antibodies after the immediate threat has passed.
If this immune contraction did not occur, our blood would be as thick as grease from all the antibodies produced against every pathogen we have encountered in our lives.
The gradual drop in the level of antibodies is one reason why people sometimes pick up infections despite vaccination —this can occur even after receivingmultiple booster doses.
The other reason is that the virus has altered itself, and some of the older antibodies are not able to lock on to the new targets.

Role of T cells in prolonged Immunity

Like B cells, which produce antibodies, T cells are central players in the immune response to viral infection.
For your immune system to fight off any kind of invader, such as a virus, you need a kind of white blood cell called a B cell, which makes antibodies, and a similar-looking white blood cell called a T cell.
T cells can play different roles altogether.
They can act as “killer cells”, attacking cells which have been infected with a virus or another kind of pathogen, or they can act as “helper cells” by supporting B cells to produce antibodies.

How do they function?

Alongside antibodies, the immune system produces a battalion of T cells that can target viruses.
Some of these, known as killer T cells (or CD8+ T cells), seek out and destroy cells that are infected with the virus.
Others, called helper T cells (or CD4+ T cells) are important for various immune functions, including stimulating the production of antibodies and killer T cells.
T cells do not prevent infection, because they kick into action only after a virus has infiltrated the body. But they are important for clearing an infection that has already started.
In the case of COVID-19, killer T cells could mean the difference between a mild infection and a severe one that requires hospital treatment.

What did the latest research find?

The researchers found that neutralising antibodies were detectable even 12 months after infection in “most individuals”.
It remained stable 6-12 months after initial infection in people younger than 60 years.
The researchers found that “multifunctional T cell responses were detected for all SARS-CoV-2 viral proteins tested”.
And most importantly, the magnitude of T cell responses did not show any difference immaterial of how severe the disease was.
While the ability of antibodies to neutralise was nearly absent against the Beta variant, it was reduced in the case of the Delta variant.

Neutralizing antibodies

SARS-CoV-2-specific neutralising antibody and T cell responses were retained 12 months after initial infection.
Neutralising antibodies to the D614G, Beta, and Delta were reduced compared with those for the original strain, and were diminished in general.
Memory T cell responses to the original strain were not disrupted by new variants.
The findings show that robust antibody and T cell immunity against SARS-CoV-2 is present in majority of recovered patients 12 months after moderate-to-critical infection.

Robustness of antibodies

The study reveals the durability and robustness of the T cell responses against variants, including Delta, even after one year of infection.
Most importantly, the robust and longstanding T cell responses were seen in people who have not been reinfected or vaccinated.
This would mean even in the absence of vaccination, a person who has been infected by the virus even one year ago would have robust immune responses.
It would offer protection against disease progressing to a severe form requiring hospitalization.