It is not known whether this blood disorder is related to the Covid vaccines. More than 31 million people in the United States have received at least one dose, and 36 similar cases had been reported to the government’s Vaccine Adverse Event Reporting System, VAERS, by the end of January. The cases involved either the Pfizer-BioNTech or Moderna vaccine, the only two authorized so far for emergency use in the United States.

But the reporting system shows only problems described by health care providers or patients after vaccination, and does not indicate whether the shots actually caused the problems.

Officials with the Food and Drug Administration and the Centers for Disease Control and Prevention said that they were looking into the reports, but that so far, rates of the condition in vaccinated people did not appear higher than the rates normally found in the U.S. population, so the cases could be coincidental. Overall, the vaccines are considered safe. A small number of severe allergic reactions have been reported, but they are treatable, and the rates are in line with those reported for other vaccines, regulators say.

Samples collected from a range of counties, and using a variety of collection methods, suggest the variant is 19% to 24% more transmissible. But in some circumstances, its advantage was much greater: In one nursing home outbreak, B.1.427/B.1.429 spread at a rate that was six times higher than its predecessors.

Researchers also discerned uniform patterns of the variant’s expansion in counties across the state. When infection rates rose, they typically did so in tandem with growing evidence of the California strain’s presence.

The variant’s enhanced propensity for spread was also evident in laboratory results. An analysis of viral samples from around the state showed that compared with people infected with other strains of SARS-CoV-2, those who were infected with the California strain had viral loads in the nasopharynx that were twice as high.

That, in turn, made it highly likely that each person infected with the new strain would go on to infect more people.

B.1.427/B.1.429’s genome includes three mutations that affect the crucial spike protein, which the virus uses to sneak into human cells and convert them into factories for its own production. One of those three mutations, dubbed L452R, affects the so-called receptor binding domain, helping the virus attach more firmly to target cells.

That adaptation has not been seen in coronavirus variants that have caused worry elsewhere.

In a UCSF lab, scientists found that the L452R mutation alone made the California strain more damaging as well. A coronavirus engineered to have only that mutation was able to infect human lung tissue at least 40% more readily than were circulating variants that lacked the mutation. Compared with those so-called wild-type strains, the engineered virus was more than three times more infectious.

In the lab, the California strain also revealed itself to be more resistant to neutralizing antibodies generated in response to COVID-19 vaccines as well as by a previous coronavirus infection.

Compared with existing variants, the reduction in protection was “moderate ... but significant,” the researchers said.

When the neutralizing antibodies went up against the homegrown strain, their effectiveness was cut in half. By comparison, when these antibodies encountered the coronavirus strain that’s now dominant in South Africa, their effectiveness was reduced to one-sixth of their usual levels. has been shown to reduce the effect of