US biotech firm Moderna on Wednesday announced initial data from a small clinical trial that showed its booster shots improved people's immune responses against key coronavirus variants of concern.

"We are encouraged by these new data, which reinforce our confidence that our booster strategy should be protective against these newly detected variants," said the company's CEO Stephane Bancel.

Forty participants were tested for their levels of neutralizing antibodies six to eight months after their primary vaccination series of two shots.

A third shot of either the original Moderna vaccine or a variant-specific booster improved antibody levels against two major variants, which were first detected in South Africa and Brazil.

The variant-specific booster performed better than the original shot, producing almost twice as many neutralizing antibodies.

The company is also testing a third type of booster, which is a combination of the other two types, and plans to announce results for it soon.

Neutralizing antibodies are proteins produced by the immune system that are custom-made to bind to a specific structure of a microbe.

In the case of the coronavirus, these are its spike proteins that dot the surface giving it its distinctive crown-like appearance.

Binding to these spikes prevents the virus from latching on to and invading our cells.

Neutralizing antibodies are therefore important first lines of defense that prevent infection.

The immune system does however contain many other key players which, especially among people who were vaccinated against the original virus or previously infected, can kick in and prevent severe disease, even if a variant breaks through and infects the host.