The anticipated summer wave of Covid-19 has arrived, with infections rising across the United States. According to the Centers for Disease Control and Prevention (CDC), Covid cases are increasing in 39 states, with no significant declines reported anywhere in the country. This surge marks the beginning of a summer wave that experts had predicted.

Although the CDC no longer tracks Covid cases directly, it estimates transmission based on emergency department visits. Both Covid-related deaths and emergency department visits have increased in the past week. Hospitalizations also climbed 25% from May 26 to June 1, the latest data available.

California, in particular, has seen a notable rise in infections. State data suggests high levels of coronavirus in wastewater, indicating increased prevalence since May. The proportion of Covid tests coming back positive in California has jumped from around 3% to 7.5% in the last month.

Dr. Thomas Russo, chief of infectious diseases at the University at Buffalo, noted, "It looks like the summer wave is starting to begin." Historically, Covid infections have spiked during the summer, partly due to increased travel and indoor gatherings where it's cooler. This year appears to be no exception, although experts expect this wave to be milder in terms of severe disease.

Several new variants are likely contributing to the nationwide trend. Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, identified variants KP.2, KP.3, and LB.1 as key players. "We're seeing the start of an uptick of infections that is coincident with new variants that are developing," he said.

These variants, descendants of the JN.1 strain that dominated last winter, are thought to be more transmissible. KP.2 became the dominant variant in the U.S. last month, followed by KP.3 in early June. Collectively known as "FLiRT," these variants account for around 63% of Covid infections in the U.S. LB.1, another rapidly growing variant, accounts for 17.5% of infections and is expected to become dominant soon.

Dr. Russo warned that LB.1 might be more infectious and better at evading immunity from vaccines or previous infections, making it a "winning formula to infect more people." Although the CDC doesn't regularly track Covid symptoms over time, illnesses caused by new variants generally exhibit consistent symptoms with those seen in the past two years.

Beyond the variants, other factors could help the virus spread this summer. As people seek refuge indoors from heat waves and gather for events like the Fourth of July, the virus may find more opportunities to transmit. Dr. Russo recommended that vulnerable individuals, such as the elderly or immunocompromised, consider getting the latest Covid vaccine if they haven't already.

For most young, healthy individuals, experts suggest waiting for the updated vaccines expected this fall. The Food and Drug Administration (FDA) has advised vaccine manufacturers to target the KP.2 variant. The CDC's Advisory Committee on Immunization Practices is set to meet soon to decide on the distribution of these updated shots.

Meanwhile, the situation in California serves as a bellwether for the rest of the nation. The state is one of 15 with high or very high coronavirus levels in sewage, reflecting a broader regional trend. Levels in California's wastewater have sharply increased since early May, nearing peaks seen last summer. In Santa Clara County, levels are considered high across Silicon Valley, while in Los Angeles County, wastewater levels have remained stable but higher than earlier in the year.

Despite the rise in cases, Covid-related deaths have remained stable, with fewer than one per day reported on average in L.A. County. The percentage of Covid tests coming back positive at medical facilities statewide continues to climb, reaching 7.5% for the week ending June 17.

As the country navigates this summer wave, federal officials are pushing for early availability of the updated Covid vaccines. Last year's rollout was complicated by a delayed release, but this year, there is hope that the vaccines will be available sooner, ideally aligning with flu shots for those who prefer to receive both vaccinations simultaneously.

Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, emphasized that while the vaccines might not be a perfect match for the variants circulating come fall, they should still provide significant protection. "The best vaccine for going into this fall season is the one that you put in your arm," he said.