A new coronavirus variant known as "Mu," identified first in Colombia in January, is now the country's predominant strain and behind its deadliest pandemic wave yet, a health official said Thursday.

The variant was responsible for Colombia's deadly third infection wave between April and June, health official Marcela Mercado told a local radio station.

During this period, with about 700 deaths per day, nearly two-thirds of tests from people who died came back positive for the Mu variant, she said.

"It is already in more than 43 countries and has shown high contagiousness," she added.

On Tuesday, the World Health Organization declared Mu, scientific name B.1.621, a "variant of interest."

It said the variant has mutations that indicate a risk of resistance to vaccines and further studies were needed to better understand it.

"The Mu variant has a constellation of mutations that indicate potential properties of immune escape," the agency said.

There is widespread concern over the emergence of new variants as infection rates tick up globally, with the highly transmissible Delta variant taking hold, especially among the unvaccinated and in regions where anti-virus measures have been relaxed.

All viruses, including SARS-CoV-2, which causes Covid-19, mutate over time and most changes have little or no effect on the properties of the virus.

But certain mutations can alter how easily a virus spreads, the severity of the disease it causes, or its resistance to vaccines and drugs.

The WHO lists four coronavirus variants of concern, including Alpha, present in 193 countries, and Delta, in 170.

Colombia recently has counted around 100 Covid-19 deaths and 2,000 infections per day, on average. 

Less than a third of Colombians have been vaccinated against the virus, which has claimed nearly 125,000 lives in the country to date.