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Omicron vs. Delta: See Which Vaccines Worked Best, Booster Effect & More in Study

Vaccine effectiveness was dramatically reduced during the omicron wave vs. the delta wave, but vaccines are still the best tool at preventing COVID infection, the CDC found -- and boosters change the entire game regardless of manufacturer

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What to Know

  • The CDC looked at COVID infections for the unvaccinated vs. fully vaccinated across the delta and omicron variant waves in 25 jurisdictions, including New York, New York City and New Jersey
  • Overall, it found vaccine effectiveness plunged during omicron vs. delta; fully vaccinated people with Pfizer had the highest weekly infection rate in December, while Moderna users had the lowest
  • COVID boosters proved to be a total game-changer; while all three drugmakers saw declining effectiveness during omicron, boosted people were much less likely to be infected than fully vaccinated people without boosters, regardless of manufacturer

New research released by the CDC Friday offers perhaps the sharpest perspective yet on three critical questions that have emerged around the COVID pandemic over the last few months.

How did the delta and omicron variant waves compare for the fully vaccinated vs. the unvaccinated? Which vaccines fared better against infection and death? And just how much added protection do vaccine booster shots really provide?

The CDC assessed data from 25 jurisdictions, including New York, New York City and New Jersey, in its analysis, over four timeframes related to the variants -- pre-delta (April to May 2021), delta emergence (June 2021), delta predominance (July to November 2021) and omicron emergence (December 2021).

The timeframes were selected based on variant proportion estimates in the United States, which the CDC acknowledges are limited in accuracy, given the relatively nominal share of cases that are genetically sequenced to isolate strains and general lags in identification, surveillance and reporting of COVID-19 variants. (No state reports more than 20% of positive samples sequenced, and the vast majority report fewer than 5% of samples sequenced, including NY (2.75%) and NJ (2.44%)

Where Did Vaccine Effectiveness Suffer Most?

This topic has been the one most publicly addressed and well supported by data evidence, including analyses recently put out by New York state. But the report put out by the CDC Friday offers a uniquely comparative look at the variant waves.

The average weekly case rates per 100,000 people for unvaccinated and fully vaccinated alike in the pre-delta wave in April to May were higher than during delta's emergence in June, with the unvaccinated still far more likely to be infected.

During the period delta became dominant, from July to November, average weekly case rates exploded for both groups -- soaring from 64 infections per 100,000 unvaccinated in the delta emergent time to 460.1 during its predominance, a seven-fold increase. Among the fully vaccinated, the rate went from 7.4 infections per 100,000 to 90.9.

In other words, vaccine effectiveness against infection fell from 93% to 89% during delta's emergence, then to 80% in the months, July to November, that delta established dominance. In December, which marks omicron's emergence, vaccine effectiveness plunged to 68%, according to the CDC.

Average weekly case rates for the fully vaccinated soared to 230.9 from 90.9 in the prior period, while they rose to 725.6 per 100,000 unvaccinated people, up from 460.1 in the prior time.

Declines in vaccine effectiveness against infection were much lower for fully vaccinated people who had a booster dose than for fully vaccinated people without one. The estimated effectiveness dropped to 80% in December for that group.

Vaccine effectiveness against death during the delta wave remained powerful, with a 94% rate reported during the months delta established dominance, in line with the 94% reported during its emergence and down slightly from pre-delta (95%). The CDC says it had insufficient data to make the same comparison for omicron.

Aaron Collins, also known as @masknerd on Twitter, explains what you need to know about high-filtration masks like KN95s, KF94s and N95s, which are able to filter out viral particles and can help you stay safe amid the spread of the Omicron variant of COVID-19.

Which Vaccines Were Most Effective in Each Wave and for Whom?

People fully vaccinated with Moderna (with no booster dose) were infected at a rate of 75 per 100,000 during delta's most dominant time, from October to November in this set of data, which was the highest effectiveness rate among the big three.

Those who completed the Pfizer regimen were infected at a rate of 93.9 per 100,000, while Johnson & Johnson recipients were infected at a rate of 107.5.

That was during delta. All three manufacturers showed dramatic drops in vaccine effectiveness against infection in December as omicron overswept the nation.

Average case rates for people fully vaccinated with Moderna and Pfizer both tripled, while Johnson & Johnson's grew by 2.2 times. Unlike during the delta wave, Pfizer recipients saw the highest infection rate per 100,000 (280.1), compared with Johnson & Johnson (246.6) and Moderna (221.6).

Fully vaccinated people aged 18 to 49 were far more likely than their older counterparts to be infected during the omicron wave, a 3.3-fold increase in risk over the prior period. Risk also increased for fully vaccinated people aged 50-64 and 65-plus in December but by lesser amounts (2.4 times and 1.6 times, respectively, based on average weekly infections per 100,000).

All three vaccines showed high effectiveness against death throughout.

How Much Did Boosters Help?

Health experts and the drugmakers themselves have cited evidence for months that their booster doses improve effectiveness against infection and severe disease significantly. The CDC data illustrates that point quite clearly.

Fully vaccinated people with boosters were infected at a rate of 25 per 100,000 between October and November, during delta, compared with an 87.7 per 100,000 average weekly case rate for fully vaccinated people without boosters.

Vaccine effectiveness even with boosters dropped markedly during the omicron wave, with the CDC reporting an average weekly case rate of 148.6 per 100,000 boosted people in December, a six-fold increase -- but boosted people were still far less likely than fully vaccinated people without boosters to get infected.

The weekly infection rate for fully vaccinated people without boosters in December was 254.8 per 100,000, 1.7 times higher than the rate for people with boosters, and three times higher than the rate for the non-boosted vaccinated group in October/November.

People boosted with Moderna were infected at a rate of 130.4 per 100,000 during omicron, 1.7 times lower than Moderna recipients without boosters. Those boosted with Pfizer were infected at a rate of 162.6 per 100,000 during omicron, also 1.7 times lower than those without boosters during the same time period.

J&J recipients who got boosters were infected at a rate of 132.7 per 100,000, a 1.8-times lower risk than J&J recipients without boosters that month.

The CDC's key takeaway from the data is clear: Vaccines worked before delta, they worked during delta and they worked during omicron.

"COVID-19 vaccines reduced risks for SARS-CoV-2 infection and COVID-19–associated death during periods of Delta variant predominance and infection risk during Omicron variant emergence," the agency said. "Substantial case rate increases were recorded among unvaccinated and vaccinated persons when Omicron became the predominant variant in December."

Case rates and vaccine effectiveness "were higher among persons who were fully
vaccinated and had received a booster dose than among fully vaccinated persons who had not received a booster dose for cases and deaths during the period of Delta predominance and for cases during the period of Omicron emergence in December," the CDC concluded. "The added benefits of booster doses were especially prominent among persons aged 50–64 and ≥65 years."

Read the full report here.

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