What Is Known about the New AY.4.2 “Delta Plus” Variant

The new AY.4.2 "Delta plus" COVID-19 variant is gaining prevalence abroad and is estimated to be slightly more transmissible.

A new strain of the coronavirus, AY.4.2, has been dubbed the “Delta plus” variant. AY.4.2 appears to be marginally more transmissible than the original Delta variant, but investigators are unsure if it will continue to grow in frequency and overtake Delta.

AY.4.2 is expanding, most notably in the United Kingdom, and is being actively monitored. As early as September 27, 2021, the UK Health Security Agency attributed up to 6% of new cases to AY.4.2. Most recently, AY.4.2 is estimated to account for 10% of new cases in England.

Because AY.4.2 is so new and concentrated in the UK, it is too early to predict whether it is more transmissible than earlier strains. However, some estimates give AY.4.2 a 10-15% transmission advantage over Delta. Experts say this is too slight to drastically change our emergency response, but it is still unknown whether AY.4.2 causes more severe COVID-19 disease than the Delta variant.

The US does not have as vigilant a sequencing system as the UK, so AY.4.2 may be flying under the radar. However, estimates indicate AY.4.2 currently accounts for less than 1% of cases and is not causing outbreaks. Unlike Delta, AY.4.2 has yet to be designated a “variant of concern” by the World Health Organization.

Delta quickly became the world’s most dominant COVID-19 strain, so experts predict that all subsequent variants, like AY.4.2, will descend from Delta.

According to Centers for Disease Control and Prevention (CDC) Director Rochelle Walensky, the organization has “identified this sub lineage here in the United States, but not with recent increase frequency or clustering, to date.”

Variants occur when a virus has time to mutate, so experts stressed the importance of getting vaccinated to protect against current and future strains of COVID-19.