On the basis of this signal

[GoodTaste]

Although genome sequencing is needed to confirm Omicron cases, some PCR tests can pick up a hallmark of the variant that distinguishes it from Delta. On the basis of this signal, there are preliminary indications that cases, although extremely low in number, are rising in the United Kingdom.
Source: Nature.

How bad is Omicron? What scientists know so far​

Does “this signal” refer back to “a hallmark”?

 

[emsr2d2]

Yes. Spotting that hallmark in a test is a signal that it's probably Omicron, not Delta.