[originally published: june 28, 2021. updated: march 1, 2022.]  note: information in this article was accurate at the time of original publication. because information about covid-19 changes rapidly, we encourage you to visit the websites of the centers for disease control & prevention (cdc), world health organization (who), and your state and local government for the latest information.  in summer 2021, people were feeling some hope—or at least cautious optimism—that the pandemic could recede to the background, although there was still the threat that new mutations of the covid-19 virus could bring it back, and it might be even stronger.  that’s when the delta variant surfaced in the united states. first identified in india in late 2020, delta swept rapidly through that country and great britain before reaching the u.s., where it quickly surged and became the predominant sars-cov-2 variant, accounting for more than 99% of covid-19 cases (at the time) and leading to an overwhelming increase in hospitalizations in some states. (omicron replaced delta as the predominant variant, but in march 2022, the cdc still included delta on its list of variants of concern in the u.s., even though it accounted for 0% of cases in the u.s. at that time.)  delta was believed to be more than twice as contagious as previous variants, and studies have shown it to be more likely than the original virus to put infected people in the hospital. people who were not vaccinated were most at risk, and the highest spread of cases and severe outcomes happened in places with low vaccination rates.  inci yildirim, md, phd, a yale medicine pediatric infectious diseases specialist and a vaccinologist, wasn’t surprised by delta's progression. “all viruses evolve over time and undergo changes as they spread and replicate,” she said.  people who are fully vaccinated against the coronavirus continued to have strong protection against covid-19 compared to those who weren’t, but there were enough questions about delta to lead the centers for disease control and prevention (cdc) to advise additional precautions, including mask guidelines regardless of vaccination status and recommendations for booster shots.  while most delta infections have been in people who have not had a vaccine, data also showed the variant to have increased transmissibility even among some vaccinated people.  here are five things you need to know about the delta variant.
5 things to know about the delta variant