Delta has all but elbowed out the three other Covid-19 variants of concern, which now represent a tiny fraction of the samples being sequenced, according to the the World Health Organization.

"Less than 1% each of Alpha, Beta and Gamma are currently circulating. It's really predominantly Delta around the world," said Maria Van Kerkhove, the WHO's technical lead on Covid-19.

Delta "has become more fit, it is more transmissible and it is out-competing, it is replacing the other viruses that are circulating," she told a WHO social media live interaction, adding that Delta had been detected in more than 185 countries to date.

All viruses mutate over time, including SARS-CoV-2, the virus which causes Covid-19 disease.

During late 2020, the emergence of variants that posed an increased risk to global public health prompted the WHO to start characterising them as variants of interest, and the more-worrying variants of concern, to inform the response to the pandemic.

The UN health agency decided to name the variants after the letters of the Greek alphabet, to avoid the countries that first detected them being stigmatised.

Besides the four variants of concern, there are also five variants of interest, but Dr Van Kerkhove said three of them - Eta, Iota and Kappa - were now being downgraded to variants under monitoring.

"This is really due to changes in circulation and that the variants of interest are just out-competed by the variants of concern. They're just not taking hold," she said.


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More Americans lost to Covid-19 than 1918 flu pandemic

Despite a century of medical advances, more Americans have now died from Covid-19 than the number who succumbed to the 1918 flu pandemic, according to new data.

The latest grim milestone comes as the country is experiencing a fourth-wave driven by the highly contagious Delta variant, with low vaccination uptake in many regions the main cause of death.

Johns Hopkins University tracker showed 675,722 US coronavirus deaths as of Friday, which surpasses the 675,000 US deaths during the influenza outbreak that began in the last year of World War I.

All told, some 50 million died worldwide in the flu pandemic, sometimes inaccurately referred to as the "Spanish flu", making it the deadliest event in human history, according to epidemiologists.

That far exceeds global Covid deaths so far - around 4.7 million.

But the US has borne a disproportionate 14% of those fatalities, despite making up only 5% of the world's population.

The US population in 1918 was less than a third of what it is now, meaning the flu deaths would be equivalent to around 2.2 million in today's terms.

Unlike today's influenzas, which impact children and the elderly the most, the 1918 flu caused unusually high mortality among young adults.

More than 600,000 white flags near the Washington Monument symbolise the lives lost to Covid-19 in the US

According to the Centers for Disease Control and Prevention, with no vaccines and no antibiotics for secondary bacterial complications, control efforts were limited in the 1918-19 to non-pharmaceutical measures.

These included "isolation, quarantine, good personal hygiene, use of disinfectants, and limitations of public gatherings," it said.

Many of the same measures, including face masks, were recommended when the Covid pandemic began.

Now, however, there are also multiple safe and highly effective vaccines that were developed and tested in record-time, but 24% of US adults, or almost 60 million, have not yet received their first dose.

Uptake has been hit by a polarised political climate and what experts call an epistemological crisis, with misinformation supercharging vaccine hesitancy to historically new heights.

Beyond vaccines, effective treatments have been developed such as monoclonal antibodies, corticosteroids to dial down hyperactive immune responses in patients with severe Covid, and advanced ventilators.