COVID-19: Researchers estimate that 73% of Americans are immune to the Omicron


The omicron wave that assaulted the United States this winter also bolstered its defenses, leaving enough protection against the coronavirus that future spikes will likely require much less – if any – dramatic disruption to society.

Millions of individual Americans’ immune systems now recognize the virus and are primed to fight it off if they encounter omicron, or even another variant.

About half of eligible Americans have received booster shots, there have been nearly 80 million confirmed infections overall and many more infections have never been reported. One influential model uses those factors and others to estimate that 73% of Americans are, for now, immune to omicron, the dominant variant, and that could rise to 80% by mid-March.

This will prevent or shorten new illnesses in protected people and reduce the amount of virus circulating overall, likely tamping down new waves. Hospitals will get a break from overwhelmed ICUs, experts agree.

“We have changed,” said Ali Mokdad, a professor of health metrics sciences at the University of Washington in Seattle. “We have been exposed to this virus and we know how to deal with it.”

The coronavirus – the current variant or future ones that are sure to pop up – remains a dangerous germ. It is still infecting more than 130,000 Americans and killing more than 2,000 every day. Tens of millions of people remain vulnerable.

And there will be future outbreaks. The notion of a “herd immunity” that could stop the virus has slipped away under the harsh reality of new variants, waning immunity, and the rejection of vaccines by some Americans.

But the coronavirus is no longer new. Two years ago it arrived in a nation where nobody’s immune system had seen it before. The entire population – 330 million people – were immunologically naive, that is, susceptible to infection.

Genomic sequencing capacity in the United States has increased in both throughput and participating laboratories during the COVID-19 pandemic, with 1,189,459 sequences submitted during June 2021–January 2022. The corresponding average of 35,431 sequences per week is approximately three times higher than the 10,643 sequences per week during the surveillance period covered by the previous report (December 2020–May 2021) (2). As of the week ending January 22, 2022, a total of 1,469,400 SARS-CoV-2 sequences met the criteria¶¶¶ for being included in national genomic surveillance estimates; 88% of sequences were from CDC-contracted commercial diagnostic laboratories, 2% from NS3, and 10% were baseline-tagged sequences. Sequences originated from 56 jurisdictions: 50 U.S. states, District of Columbia, American Samoa, Guam, Northern Mariana Islands, Puerto Rico, and U.S. Virgin Islands.

During June 2021, the proportion of several variants changed markedly (Figure 1). Alpha (B.1.1.7 and Q sublineages) continued to decline nationally. Gamma (P.1 and descendent lineages) peaked at 12.1% (95% CI = 9.8%–14.7%) during the week ending June 5, 2021, before declining; Mu (B.1.621) and Lambda (C.37) increased to their peaks of 4.5% (95% CI = 3.5%–5.6%) and 0.6% (95% CI = 0.3%–0.9%), respectively, for the week ending June 19, before declining as Delta (B.1.617.2 and AY sublineages) reached predominance (>50%).**** The overall effect was a reduction in SARS-CoV-2 variant diversity because of Delta’s growth in proportion, with five variants being monitored circulating at >1% in June and only one variant circulating above this threshold in September. The Delta variant rose from 1% of circulating SARS-CoV-2 viruses nationally during the week ending May 1, to >50% by the week ending June 26, and to >95% by the week ending July 31. Delta prevalence was >95% in all 10 HHS regions†††† by the week ending July 31 and remained >50% in each region for ≥24 weeks.

The figure is an area graph illustrating national weekly proportion estimates of SARS-CoV-2 variants in the United States from January 2, 2021, through January 22, 2022.
Abbreviations: NS3 = National SARS-CoV-2 Strain Surveillance program; PANGO = Phylogenetic Assignment of Named Global Outbreak; WHO = World Health Organization.
* Sequences are reported to CDC through NS3, contract laboratories, public health laboratories, and other U.S. institutions. Variant proportion estimation methods use a complex survey design and statistical weights to account for the probability that a specimen is sequenced.
 SARS-CoV-2 WHO variant label and PANGO lineage: Alpha (B.1.1.7); Beta (B.1.351); Gamma (P.1); Delta (B.1.617.2), Epsilon (B.1.427/B.1.429); Zeta (P.2); Eta (B.1.525); Iota (B.1.526); Kappa (B.1.617.1); Lambda (C.37); Mu (B.1.621); and Omicron (B.1.1.529).

The Omicron variant proportion rapidly increased after the first U.S. case was reported on December 1 (4). Omicron first accounted for >1% of circulating lineages nationally during the week ending December 11, 2021, >50% of viruses for the week ending December 25, and >95% by the week ending January 8, 2021. As of the week ending January 22, 2022, national genomic surveillance estimates were 99.2% (95% CI = 99.0%–99.5%) for Omicron and 0.7% (95% CI = 0.5%–1.0%) for Delta. Region 7 had the highest proportion of Delta (3.0%; 95% CI = 1.9%–4.4%) and the lowest proportion of Omicron (97.0%; 95% CI =95.6%–98.1%). Region 9 had the highest proportion of Omicron (99.8%; 95% CI = 99.6%–99.9%) and the lowest proportion of Delta (0.2%; 95% CI = 0.1%–0.4%). Omicron’s variant proportion had an estimated initial doubling time of 3.2 days (95% CI = 3.1–3.4 days), which was faster than those of Delta (7.2 days; 95% CI = 7.0–7.4 days), Alpha (11.0 days; 95% CI = 8.3–16.1 days), Gamma (13.1 days; 95% CI = 12.0–14.3 days), and Mu (14.7 days; 95% CI = 13.8–15.7 days). Omicron rose from 1% to 99% of infections nationally in 6 weeks, compared with 18 weeks for Delta (Figure 2).

The figure is a line graph illustrating estimated variant proportions with 95 percent CIs over the first 14 weeks of each variant’s emergence (from the time of exceeding 1% of national circulating viruses) for six SARS-CoV-2 variants in the United States from November 2020 through January 2022.
Abbreviations: NS3 = National SARS-CoV-2 Strain Surveillance program; PANGO = Phylogenetic Assignment of Named Global Outbreak; WHO = World Health Organization.
* 95% CIs for estimates are shown by shaded areas. Sequences are reported to CDC through NS3, contract laboratories, public health laboratories, and other U.S. institutions. The methods for estimating variant proportions and 95% CIs use a complex survey design and statistical weights to account for the probability that a specimen is sequenced.
 SARS-CoV-2 WHO variant label and PANGO lineage: Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), Delta (B.1.617.2), Mu (B.1.621), and Omicron (B.1.1.529).

“I am optimistic even if we have a surge in summer, cases will go up, but hospitalizations and deaths will not,” said Mokdad, who works on the Institute for Health Metrics and Evaluation model, which calculated the 73% figure for The Associated Press.

With varying degrees of relief and caution, many Americans are starting to return to their pre-pandemic lifestyles.

Sarah Rixen, 41, of Bismarck, North Dakota, started singing again with a civic chorus after taking a year off. Now, with omicron winding down, she said she feels more confident than at any time since the crisis began.

“But I am still a little leery that there could be another variant around the corner,” said Rixen, noting that her family and most of her relatives are fully vaccinated. “I am still going to wear a mask.”

As mask mandates ease, workers return to offices and flights fill up, experts are trying to understand whether this return to normal can last, or if another setback is looming.

To address that, researchers are trying to answer questions about the virus, the vaccine, and how our bodies respond: How fast is booster protection waning against omicron? How long does protection from infection last? How many mild infections were never reported? How many people got infected but had no symptoms?

To find clues, they use health data from other countries such as Britain, Denmark, South Africa and Qatar to project what could be in store.

Scientists at Johns Hopkins University Bloomberg School of Public Health estimates that about three out of four people in the United States will have been infected by omicron by the end of the surge.

“We know it’s a huge proportion of the population,” said Shaun Truelove, an epidemiologist and disease modeler at Johns Hopkins. “This varies a lot by location, and in some areas we expect the number infected to be closer to one in two.”

That means different regions or groups of people have different level of protection – and risk. In Virginia, disease modelers are thinking about their population in terms of groups with different levels of immunity.

They estimate about 45% of Virginians have the highest level of immunity through boosted vaccination or through vaccination plus a recent infection with omicron. Another 47% have immunity that has waned somewhat; and 7% are the most vulnerable because they were never vaccinated and never infected.

In all, the vast majority of Virginians have at least some immunity, said Bryan Lewis, a computational epidemiologist who leads University of Virginia’s COVID-19 modeling team.

“That’s going to be a nice shield of armor for our population as a whole,” Lewis said. “If we do get to very low case rates, we certainly can ease back on some of these restrictions.”

Still, while the population is better protected, many individuals are not. Even by the most optimistic estimates for population immunity, 80 million or so Americans are still vulnerable. That’s about the same as the total number of confirmed infections in the U.S. during the pandemic.

“The 26% who could still get omicron right now have to be very careful,” Mokdad said.

Andrew Pekosz, a virus researcher at the Johns Hopkins Bloomberg School of Public Health, is concerned that people—particularly unvaccinated omicron survivors—may have a false sense of security. “In an ideal world, unvaccinated individuals infected with omicron would be lining up for a vaccine shot,” he said.

Also, estimating protection is far from an exact science. It’s a moving target, as immunity wanes and new variants circulate. Protection varies widely from person to person. And it’s impossible to know for sure how many people are protected at all. The IHME model estimates a wide range—from 63% to 81% of Americans.

“We’ve reached a much better position for the coming months, but with waning immunity we shouldn’t take it for granted,” Mokdad said.



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