A Couple More Pieces to the Omicron Puzzle

When I’m looking at the covid data I feel like I’m putting a jigsaw puzzle together with half the pieces missing, trying to assemble a halfway coherent picture out of fragments.

The CDC just came out with analyses of vaccine effectiveness that are consistent with the state-specific US numbers I previously compiled from NY, MA, OR, etc. In the US:

  • 2 shots of vaccine are 57% effective at preventing Omicron hospitalizations and 42% effective at preventing ER visits.
  • The booster increases effectiveness to 90% at avoiding hospitalization and 83% at avoiding the ER.
  • Compared with a 2-shot vax, the booster reduces the likelihood of symptomatic Omicron infection by 58% to 66%.

At 4 am I realized why the UK data I summarized yesterday look less rosy — in all likelihood it’s because there aren’t any unvaxxed Brits who’ve not already been infected. The UK Office of National Statistics estimated that, by the end of 2020, 98% of the UK population had covid antibodies, either from vaccines or infections or both. So when the UK vax effectiveness studies say that 2 shots don’t improve one’s odds of avoiding covid infections, it’s because the double-vaxxed are being compared with people (like Djokovic) who’ve gotten a good dose of covid immunity from a prior infection. So, 2 shots are about equivalent to 1 prior infection in preventing symptoms, and quite a bit more effective at preventing hospitalization. But there aren’t any “covid virgins” left in the UK to measure how much protection that amounts to.

Here’s how I’m putting the puzzle together with the scattered pieces I’ve assembled:

  • Omicron is more contagious than any prior variant, but also less virulent.
  • If someone gets infected by Omicron, they likely won’t have symptoms, but if they do, they’re very unlikely to wind up hospitalized or dead.
  • A 2-shot vax offers protection against Omicron, especially against severe illness.
  • Adding the booster adds a lot more protection.

Vaccine Effectiveness Against Omicron: UK Data

From the UK Health Security Agency, Technical briefing 34, 14 January 2022, pages 26-27.

Effectiveness of Pfizer vaccine at preventing Omicron symptomatic disease:

  • 2 doses –> 10% effectiveness at 20 weeks post-vax
  • 2 doses + Pfizer booster –> 55% effectiveness at 10+ weeks
  • 2 doses + Moderna booster –> 65% effectiveness at 5-9 weeks

Effectiveness at preventing Omicron hospitalization, all vaccines: (including the less effective AstraZeneca):

  • 2 doses –> 44% effectiveness at 25+ weeks
  • 2 doses + booster –> 83% effectiveness at 10+ weeks

The UK vax effectiveness numbers against Omicron are lower than the state-specific US calcs I’ve noted here and here. The UK has higher vaccination and booster rates than the US. It’s possible that, due to the politically charged and class-stratified responses to covid in the US, those who get vaxxed are also more likely to benefit from other precautions against contagion: working from home, wearing masks, avoiding superspreader events, etc.

Most notably, the UK report presents separate effectiveness rates for 2-shot versus 3-shot vaccinations. Clearly getting the booster provides much improved protection, even against Omicron.