'Stealth' Meaning Explained As Stealth Omicron BA.2 Variant Found in 26 States

The Omicron BA.2 sub-type continues to attract attention as it spreads in various parts of the world including the U.S.

In several news reports and even by some healthcare experts, BA.2 has sometimes been referred to as "stealth Omicron."

While the terminology has picked up recently in terms of usage, Google Trends data shows that the term was being used online as far back as early December. This is around the same time as BA.2 was designated as a sub-variant on December 6.

Not all experts agree on the usage of the term. Tom Peacock, a virologist at Imperial College London, tweeted last week that BA.2 "was misdescribed as a 'stealth variant' early on."

According to Peacock, the likely reason why BA.2 has picked up the "stealth Omicron" nickname is because it is detected differently in certain COVID PCR tests compared to the more dominant version of Omicron BA.1.

Shortly after Omicron first emerged, scientists noted that it lacked one of the certain mutations that COVID tests were looking for. To use the more scientific term put forward by the U.K. Health Security Agency, BA.2 "does not have the spike gene deletion at 69-70 that causes S-gene target failure (SGTF)".

S-gene target failure, also called S-gene dropout, is what happens when a PCR test cannot detect this mutation. Scientists actually used this to their benefit with Omicron and were able to detect the variant at faster rates than previous surges in infection because S-gene target failure could be used as a sort of marker for Omicron.

Usually, scientists carry out genetic analysis to determine which COVID variant a person is infected with.

With BA.2 however, the S-gene target failure no longer occurs. This is because it does not have the same genetic mutation present in earlier Omicron samples.

It should be noted that BA.2 is still detectable, and the "stealth Omicron" nickname "doesn't mean undetectable by our PCR COVID tests," microbiologist S. Wesley Long tweeted last week.

It means that while a PCR test will still tell if someone has COVID, it may not tell them that they have BA.2 specifically and genomic testing would have to be carried out to determine that.

"It's not that the test doesn't detect it; it's just that it doesn't look like Omicron," Long told ABC News on Tuesday. "Don't get the impression that 'stealth Omicron' means we can't detect it. All of our PCR tests can still detect it."

BA.2 has been detected in more than 100 COVID cases across 26 states in the U.S., according to the viral genome database GISAID as of early Wednesday morning ET—an increase from 92 cases on Tuesday morning. It has also been found in nearly 50 countries worldwide according to Outbreak.Info, which uses GISAID data.