Mystery as to why new COVID variants have stalled in growth

Throughout much of the pandemic, there has been a constant shifting in terms of which COVID-19 variants are most dominant, at a given time, in the U.S.

However, for the last five weeks, federal data shows that there has been little to no growth in the different proportions of COVID-19 variants in the country.

For more than nine months, the omicron variant, and its subvariants, have been dominant in the U.S. But now, health experts say it is unclear why the growth of the omicron strains appears to have stagnated, or why it is that no other significant variants have emerged to challenge its dominance.

"Unlike previous variants, BA.5 appears to have more staying power. A mix of higher transmissibility, waning immunity and relaxed restrictions likely contribute to the ability of this variant to find more hosts to infect," said John Brownstein, Ph.D., an epidemiologist at Boston Children's Hospital and an ABC News contributor.

BA.5 is currently estimated to account for about 88.6% of new COVID-19 infections -- a share that has plateaued over the last five weeks, according to updated data from the Centers for Disease Control and Prevention.

BA.4 currently accounts for an estimated 2.8% of new cases in the U.S., and a recently identified subvariant, BA.4.6, is estimated to account for 8.4% of new cases -- up slightly from last week when the subvariant accounted for 7.6% of new cases.

Combined, the BA.4 and BA.5 subvariants account for nearly 100% of new cases in the U.S., according to the CDC data.

The BA.4 and BA.5 subvariants appear to have a transmission advantage over the original omicron strain, according to the World Health Organization, and thus, scientists have been closely monitoring the increase in reported cases. At this time, it does not appear as though BA.5 has an increase in severity.

The slowing of the omicron strain's growth pattern leaves many questions unanswered as to whether there will be viral resurgence in the fall and winter.

"We still have open questions as to what this means for a fall surge and the opportunities for a new variant to displace it," Brownstein said.

The stagnation of growth with the omicron variants comes as the U.S. appears to experience a parallel plateauing of new COVID-19 infections. The nation had been reporting consistent declines throughout the late summer, but in recent weeks, that number has been hovering around 84,000 new cases each day, according to the CDC.

As previously reported, dozens of states have moved to shutter public testing sites, with more at-home COVID-19 tests now available. Most Americans are not reporting their results to officials, and thus, experts suggest that infection totals are likely significantly undercounted.

Just over 400,000 tests are being reported each day, marking the lowest number of confirmed tests since the onset of the pandemic.

Although new case rates are still dropping in parts of the West, across areas of the Northeast, the Midwest and even parts of the South, case rates have plateaued at a high level or are showing signs of increasing again.

Hospital admission levels also appear to be plateauing nationally. About 5,100 virus-positive Americans are entering the hospital each day, down by about 3.7% in the last week, according to CDC data.

Death rates also remain persistently high, with hundreds of Americans still losing their lives to the virus each day. According to the CDC, the average number of daily COVID-19-related deaths remains more than 400 deaths reported each day.

Thousands are still losing their lives every week, and over the last seven days alone, the U.S. has reported more than 2,800 deaths -- still one of the highest weekly totals in months.