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Infectious disease experts are weighing the need for booster shots of the Pfizer/BioNTech or Moderna mRNA-based vaccines for Americans who received Johnson & Johnson's (JNJ.N) one-dose vaccine due to the increasing prevalence of the more contagious Delta coronavirus variant. A few say they have already done so themselves, even without published data on whether combining two different vaccines is safe and effective or backing from U.S. health regulators. Canada and some European countries are already allowing people to get two different COVID-19 shots. (Erman, 6/27)

The Delta coronavirus variant is now the third-most common in California, new data show, underscoring the danger of the highly contagious strain to people who have not been vaccinated against COVID-19. The variant makes up 14.5% of California coronavirus cases analyzed so far in June, up from 4.7% in May, when it was the fourth-most identified variant in California, according to data released by the California Department of Public Health. (Lin II, Money and Wigglesworth, 6/27)

The Delta variant, a strain of Covid-19 believed to be more transmissible and dangerous than others, is likely to break out in some US communities, a health expert told CBS's Face the Nation. "It's not going to be as pervasive," Dr. Scott Gottlieb, former commissioner of the US Food and Drug Administration, told the station Sunday. "It's going to hyper-regionalized. There's certain pockets of the country where you're going to have very dense outbreaks." Those pockets will be ones with low vaccination rates and low rates of prior infection, Gottlieb said, like in many rural and southern communities. (Holcombe, 6/28)

Based on a recent government projection, the Delta variant of the Covid-19 virus could become the dominant type of this coronavirus in the U.S. within a month, making it one of most aggressive variants to take hold in the country. Delta is the latest in a series of variants that have spread throughout the U.S. Like all viruses, coronaviruses mutate as they reproduce. Some of these genetic changes make them better at infecting human cells or evading our immune defenses. As newer, better-adapted variants emerge, they push aside earlier versions of the virus. Here is a look at how this process has played out across the U.S. since the start of the pandemic. (Ulick, 6/27)

Since the start of the coronavirus pandemic, the U.S. has been keeping a close eye on the U.K. From its initial response to Covid-19 (questioned by many), to its much-praised immunization program and world-class research, all have helped inform how the U.S. — which faced its first major Covid outbreak after Britain — has reacted. (Ellyatt, 6/28)

The “delta variant” has come to dominate headlines, having been discovered in India where it provoked an extreme surge in Covid-19 cases before spreading around the world. But now a mutation of that variant has emerged, called “delta plus,” which is starting to worry global experts. (Ellyatt, 6/24)