• New strains of Covid-19 are spreading globally, raising concerns among health professionals about a possible summer spike in cases.
• The FLiRT variants have been on the rise in the U.S. and Europe as the coronavirus continues to mutate from earlier strains.
• KP.2 is now the dominant strain in the U.S., according to the latest data from the Centers for Disease Control and Prevention.

LONDON — New strains of Covid-19 are spreading globally, raising concerns among health professionals about a possible summer spike in cases, four-and-a-half years since the onset of the pandemic.

The FLiRT variants — whose label derives from the names of the mutations in the variants' genetic code — have been rising in the U.S. and Europe as the coronavirus continues to mutate from earlier strains.

The new grouping are descendants of the previously dominant JN.1 variant, an offshoot of omicron. There is currently little evidence that the new strains are more severe, but they appear to have independently picked up the same set of mutations, according to John Hopkins Bloomberg School of Public Health.

KP.2 is now the dominant strain in the U.S., according to the latest data from the Centers for Disease Control and Prevention. The strain accounted for 28.2% of all cases in the two weeks to May 11, up from 3.8% at the end of March, shortly after the strain was first discovered.

Cases of KP.1.1, another FLiRT variant, have also risen to account for 7.1% of current infections, the agency said.

In Europe, too, cases have been rising, with the new variant now detected in 14 countries.

The World Health Organization in its latest update earlier this month said that cases remain limited in all reporting countries. Individual countries, however are showing "slight increases in detections from very low levels."

Last week, the U.K. Health Security Agency said it is continuing to monitor data relating to the new variants in the U.K. and internationally, assessing their severity and the ongoing effectiveness of vaccines. "There is no change to the wider public health advice at this time," the agency said in an update.

It currently seems unlikely that the new strains will cause a major wave of infections as seen in the past when public immunity was lower, said Jennifer Horney, professor of epidemiology at the University of Delaware. But she noted that the new strains would likely lead to an uptick in cases during the coming summer months.

"While our idea of what a wave of COVID-19 infections looks like has changed over the course of the pandemic, it is likely that these new strains will cause increases in the number of cases in the U.S. over the next few months," Horney told CNBC via email.

"Many will be mild, based on our existing immunity and not the changes to the circulating strain," she said.

Still, health professionals will be watching closely to see how effective current vaccines are against the new strains.

Next month, the U.S. Food and Drug Administration's vaccines advisory committee will meet to discuss recommendations for the variant mix for this winter's Covid-19 vaccine, having postponed an earlier discussion to collect more data.