Faced with an increase in hospitalizations, the American health authorities insisted on Tuesday on the importance of vaccines against severe forms of COVID-19, even if the sub-variants of Omicron seem to escape the immune response.

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The United States is seeing some 5,100 COVID-19-related hospitalizations every day, "double the admissions in May," noted Centers for Disease Control and Prevention (CDC) director Rochelle Walensky. during a press briefing.

This increase is linked to the meteoric rise of the Omicron BA.4 and BA.5 subvariants, first detected in April and which represent respectively 16% and 65% of the virus currently circulating in the United States.

Although they don't appear to be more dangerous than other variants, "they are more contagious and evade the immune response more," whether acquired through vaccination or through previous infection, Walensky explained.

But the effectiveness of vaccines against severe forms and death remains "probably high for BA.4 and BA.5", she added, calling on Americans to receive all the recommended doses as soon as possible.

Faced with these sub-variants, "we must not panic or let them upset our lives, but we must take them seriously," added immunologist Anthony Fauci, President Joe Biden's main adviser on the health crisis.

"If you have not done your boosters, as recommended", at least a second booster dose for those over 50, "you are facing a higher risk", he added, while many Americans, out of vaccine fatigue or because they expect a new, more targeted generation in the fall, delay receiving these injections.

The United States has between 100,000 and 150,000 new reported cases of COVID-19 per day -- an underestimated number due to the expansion of rapid home tests, the results of which are not always reported to authorities.

The country, which has deplored more than a million deaths since the start of the pandemic, still records 300 to 350 deaths every day.