Breaking: Pfizer/BioNTech vaccine prevents most Omicron Covid-19 hospitalizations.

Newly released data show the effectiveness of the two-dose series dropped somewhat from the Delta period, but still performed impressively.

Researchers in South Africa have published data in the New England Journal of Medicine showing that the two-dose Pfizer/BioNTech vaccine remains 70% effective against Covid-19 hospitalization, even in the face of the Omicron Variant. While Omicron has been found in several other studies to cause many more infections among the vaccinated and boosted (let alone the unvaccinated), these new findings indicate that the implications of these infections remain far less dire than they were during the pre-vaccine era. In other words, the vaccines have held up to their greatest challenge yet. As expected, a majority of Omicron Covid-19 hospitalizations in South Africa occurred among the unvaccinated.

Diving into the details of the study, we see that across the board, the rates of Covid-19 hospitalization during the Omicron period were lower than during the Delta period. Depending on the age group, hospitalization rates ranged from 2-to-6 times lower during the Omicron period than during the Delta period. However, most of these findings appear to reflect a combination of increased rates of vaccination in the last few months in South Africa (where vaccination rates remain low, with just over 25% of the population fully vaccinated, though now steadily rising) and a lower average age among newly infected individuals during the Omicron period.

For example, younger adults represented a higher fraction of hospitalizations in the last month compared to previous waves. However, younger adults are also a much less-vaccinated demographic, including in South Africa. That’s why it will only be after these “confounding variables” are factored out that we will know whether Omicron is inherently milder than Delta. That said, while exact figures can’t be triangulated, the new data from South Africa strongly imply that the explanations for reports of milder disease during the Omicron outbreak hinge more on the characteristics of the targets (average age and vaccination status) among the new cases detected, and less about the biology of the new variant. Therefore, how much the biology of the Omicron variant itself is responsible for lower rates of hospitalization remains unknown and speculative.

Some data that were not published—but which were shared with Inside Medicine by the research team that published this latest study from South Africa—show an important age-specific pattern: among individuals ages 18-29, vaccine effectiveness against Omicron hospitalization was found to be 92%. Among individuals ages 30-39, 40-49, 50-59 vaccine effectiveness was somewhat lower but still highly impressive, at 75%, 82%, and 74% respectively. However, vaccine effectiveness against Omicron-related hospitalization was another notch lower in older groups, with vaccine effectiveness pegged at 67% among individuals ages 60-69 and 59% among those ages 70-79 (Data for 80+ were not specified).

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Lower vaccine effectiveness against Omicron in older populations is cause for concern. These findings clearly suggest, yet again, that booster doses for people in older age groups (and other high-risk groups such as those with immune compromise) stand to improve outcomes meaningfully. That also means that outcomes in the United States may be better than elsewhere, since around 57% of the fully vaccinated US population ages 65 and older have already been boosted. Just how much added protection against severe disease and hospitalization those booster doses provide on top of these new figures from South Africa remains undetermined. But again, the effects will likely be age-dependent, with the most benefit coming to those who are the most vulnerable.

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The data are complicated. The deeper we get into the pandemic, the harder the questions are. The science is no longer so straight-forward that anyone who took high school biology and math can just eyeball it, which in essence was the case last year). Of course, I’ll continue to assess the new data and provide you with updates.

But fortunately, the main message remains uncomplicated and unchanged: we must vaccinate everyone who is eligible (both in the US and around the world), and boost the highest risk groups first. With respect to their risks, I fear that in the US, we’ve under-boosted the demographics who need added protection the most, including seniors and those living in nursing homes, while providing millions of booster doses to young healthy people in whom added benefits remain smaller and difficult to measure, beyond a short-term decrease in mild infections.

Have we done enough? Clearly not. But if we had not had as successful of a vaccine rollout as we did in 2021 in the United States (imperfect and incomplete as it was, and is), things would be much, much worse.

Omicron is testing us. So far, our grade is neither an A nor is it an F; it’s incomplete.

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Special thanks to Dr. Glenda Gray and Shirley Collie for providing unpublished data from South Africa described here.