The initial message was “get fully vaccinated, and we’ll hopefully get back to normal soon.” It’s true that a single dose was pretty good (although the protection was probably short-lived) but there was never official advice to get a single dose of anything but J&J. It was always 2.
When Delta came around, they said “a single dose no longer gives much protection. But two doses recently, or two doses a while ago plus a booster, gives excellent protection”.
A small South African study showed that neutralization was 40-fold less effective against omicron than against wild-type. That’s a big reduction in effectiveness, but vaccination still provides a real benefit. So more antibodies are likely to help
https://www.medrxiv.org/content/10.1101/2021.12.08.21267417v1?ijkey=a3df161dedeb78f6f26f9f357538b9a07ad417bb&keytype2=tf_ipsecsha
Pfizer found that a third dose increases antibodies 25-fold
https://www.pfizer.com/news/press-release/press-release-detail/pfizer-and-biontech-provide-update-omicron-variant
(Note that a factor of 2 is a fairly small difference in effectiveness, typically.) So a third dose restores a lot of the lost effectiveness.
The best data I’ve seen on boosters for Omicron at the moment is from the UK, which spaced the first two doses. (No doubt better data will emerge; there’s very little data on omicron just yet.) This is actual infection data, not just counting antibodies. The immune system is a lot more complex than just antibodies, so this is superior to the antibody studies above, although preliminary, of course:
https://khub.net/documents/135939561/430986542/Effectiveness+of+COVID-19+vaccines+against+Omicron+variant+of+concern.pdf/f423c9f4-91cb-0274-c8c5-70e8fad50074
And here’s the primary result:
Among those who had received 2 doses of ChAdOx1 [Oxford-AstraZeneca], there was no protective effect of vaccination against symptomatic disease with Omicron from 15 weeks after the second dose. Among those who had received 2 doses of BNT162b2 [Pfizer-BioNTech], vaccine effectiveness was 88.0% (95%CI 65.9 to 95.8%) 2-9 weeks after dose 2, dropping to 48.5% (95%CI: 24.3 to 65.0%) at 10- 14 weeks post dose 2 and dropping further to between 34 and 37% from 15 weeks post dose 2. Among those who received ChAdOx1 as the primary course, from 2 weeks after a BNT162b2 booster dose, vaccine effectiveness increased to 71.4% (95%CI: 41.8 to 86.0%). Vaccine effectiveness increased to 75.5% (95%CI: 56.1 to 86.3%) after the booster among those who had received BNT162b2 as the primary course.
Summary: Protection from symptomatic Omicron is decent from recent completion of two doses of Pfizer, or from recent boosting with a third dose of Pfizer (75%+). Protection from AZ more than 15 weeks ago was nil, but that increased to 71% after a Pfizer booster.
They only analyzed 581 total cases of omicron, so they didn’t have enough data to determine the efficacy of Moderna, nor the efficacy of any vaccine against serious illness.
All that being said, the theory predicts that vaccine (and prior infection) should protect better against serious disease than against symptomatic disease. You need neutralizing antibodies to protect against symptomatic disease, but trained T-cells protect against serious disease, and T-cell responses tend to be less specialized and more flexible in the face of viral mutations than the antibodies you have knocking around.
So if you are low-risk, and don’t have anyone in your life you are trying to protect from catching covid from you, you can probably wait until we get an omicrom-specific vaccine. Me, I’m old. I will take all the protection I can get. (And I care for my immune-compromised mother.)