But the variant, called C.1.2, isn't rapidly circulating worldwide yet, which means Delta still has the upper hand, scientists say.
Current vaccines still protect against severe COVID-19 cases, hospitalizations, and death, but experts fear what might happen in the future.
Insider spoke with three people who received unauthorized booster shots in the US, and one who received an authorized booster in Israel.
It's not yet clear whether Delta Plus is more transmissible than Delta. Vaccines may still hold up well against it.
The Delta variant makes it easier for vaccinated people to transmit the virus, CDC director Rochelle Walensky said.
Verknipt festival near Utretcht became a symbol of the Netherlands' resurgent coronavirus outbreak, which has worsened since restrictions were lifted.
Arkansas, Mississippi, Missouri, and Wyoming have seen upticks in daily coronavirus cases and hospitalizations.
Arkansas still has one of the lowest vaccination rates in the US, due in large part to widespread vaccine hesitancy.
Lambda was declared a variant of interest by the WHO last month because of its unusual set of mutations.
Speaking on Thursday, CDC Director Rochelle Walensky said unvaccinated people are "particularly at risk for severe illness and death."
A single dose of Pfizer's or AstraZeneca's vaccines - both of which require two shots - was either weakly or not at all effective against Delta.
Lambda was identified as a variant of interest by the WHO on June 15. It is dominant in Peru, but not seen much elsewhere.
The Delta variant didn't exist in the US until March - and in the space of a few months appears to have become the most common variant.
Johnson & Johnson said its COVID-19 vaccine produced a "strong" antibody response against the Delta variant, citing two small studies.
Speaking to CNN on Wednesday, Dr. Vivek Murthy contrasted them with fully vaccinated people, who are largely protected against the Delta variant.
Israel and the UK have seen a rise in cases, but neither got the sharp increase in deaths seen in places that have been slower to roll out vaccines.
Some experts, fearing Delta, are having Pfizer booster shots after J&J's one-dose vaccine. "We shouldn't wait" for official recommendations, one said.