Two doses of the COVID-19 vaccine are probably not enough.

That's one of the big conclusions being drawn from various virus research labs in recent days, a finding that's raised alarm bells in government seats around the globe. On December 12, British Prime Minister Boris Johnson gave an emergency address to announce the raising of the country's COVID-19 alert level. The upshot: the United Kingdom now plans to offer all Britons 18 and older a third dose of a vaccine by the end of the year.

The news that the Omicron variant can partially evade protection from what, until recently, was considered the gold standard—the two-jab regimens of the Pfizer-BioNTech or Moderna vaccines—comes from the preliminary results of a U.K. study, and others done in South Africa, Israel, Hong Kong, Sweden and by Pfizer itself, which show the number of virus-fighting antibodies found in a person's blood after exposure to the Omicron variant was far less than with previous variants.

While studies found varying degrees of reduction in vaccine effectiveness, they all indicated two doses were not as effective as three when it came to Omicron. One of the studies, from South Africa, found that the two-jab efficacy of the Pfizer-BioNTech vaccine—which began in the low 90% range but fell to around 50%—dropped to a mere 22.5% against the Omicron variant.

Here are what the results from the studies mean.

What does it mean to be 22.5% effective?

The South African study from the Africa Health Research Institute in Durban found a two-shot course of Pfizer's vaccine may have just 22.5% efficacy against symptomatic infection with the Omicron variant. This came after the drugmaker announced preliminary findings of a 40-fold decrease in neutralizing antibodies produced by people with two shots of the shot compared with the first strain of COVID-19.

However, Alex Sigal, head of research at the South African laboratory noted that while there is a diminished effectiveness of vaccines, the two-shot regimen will still protect against severe disease because "Omicron escape [is] incomplete."

How much does a booster dose help?

A study by the University of Oxford published on December 13 found three doses of Pfizer offers 75.5% effectiveness against symptomatic disease, and two doses of AstraZeneca followed by a Pfizer booster had 71.4% effectiveness against the Omicron variant.

Paul Bieniasz, a professor of virology at the Rockefeller University, found that a booster shot on top of a prior infection or double vaccination produces 30-to-200 times more neutralizing antibodies. Bieiasz tweeted, "It is time to discard the notion that two doses of mRNA means 'fully vaccinated.' It is time to discard the notion that prior SARS-CoV-2 infection means you don’t need to be vaccinated."

How does the Omicron effect vary by vaccine?

There is little Omicron data on vaccine effectiveness when compared across manufacturers. In the U.K., two of the most popular vaccines, Pfizer and AstraZeneca have been comparatively tested to see if they produced lower levels of antibodies with a Pfizer booster, but only data has been released on the effectiveness of two shots of Pfizer without a booster.

Little is known about the efficacy of a double-shot of AstraZeneca without a booster, which was used extensively in the U.K. at the start of the vaccine roll out.

Why so much worry in the U.K.?

Cases of the Omicron variant are shooting up in the U.K., with 1,239 new cases detected on December 12—almost double the 633 cases reported the day before—and it has just reported its very first death from the new variant on December 13, Johnson announced. Around 45.9 million people, or 80% of the population, has received two doses of a vaccine while 23 million have been given booster shots.

At the current pace in Britain, Omicron is going to become the dominant strain of COVID-19 by mid-December, the government said, with “at least 50% of COVID-19 cases to be caused by the Omicron variant in the next two to four weeks,” the Health Security Agency said.

The steep rise in cases has roiled the foreign exchange markets, with the British pound sterling falling 0.3% against the dollar.