What are the differences between omicron and BA.2?
Dr. Huerta responds 4:14
(CNN) --
With a new version of the omicron variant of coronavirus gaining traction in the United States, as many as 28 million older Americans remain at risk of severe illness from Covid-19, either because they are unvaccinated or only partially vaccinated. or because more than five months have passed since their second or third dose of a vaccine, according to a CNN analysis of federal data.
As the United States warily watches the increase in cases caused by the BA.2 subvariant in Europe, the immune status of adults older than 65 years will be a key indicator of how future variants affect the United States because it dramatically increases the risk of severe outcomes with age.
"You're really looking at that older age group and how much immunity they have, whether it's from previous infection or vaccination, which I think has been the best indicator so far of how serious a given number of cases will end up being in terms of hospitalizations and deaths. said Stephen Kissler, who specializes in infectious disease modeling at the Harvard TH Chan School of Public Health.
An analysis by the UK Health Security Agency shows that the BA.2 subvariant of omicron is about 80% more contagious than the BA.1 variant, the virus that caused the latest wave of infections in the United States over the winter. .
Cases and hospitalizations are on the rise in the UK and several other European countries where BA.2 has become dominant.
Although direct comparisons with BA.1 indicate that BA.2 is not more likely to lead to hospitalization, this variant has the potential to overwhelm health care resources in the United States once again if it finds enough vulnerable people to infect.
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The group most vulnerable to the BA.2 variant
The group most vulnerable to this variant are adults over 65 years of age, especially those who have little immunity against the virus.
This is why Pfizer and BioNTech this week petitioned the US Food and Drug Administration (FDA) to green-light the fourth dose of the vaccine for older adults.
"It's that group that's the most problematic when it comes to serious, critical and fatal illnesses. It doesn't mean younger people don't end up in the hospital sometimes; it's just not at the same rate," said Jeffrey Shaman, who specializes in in modeling the spread of infectious diseases at Columbia University's Mailman School of Public Health.
Shaman points to Hong Kong, which is in the midst of a severe wave caused by BA.2.
and it has the highest death rate from covid-19 in the world.
"And they haven't seen the worst of it because it's a little bit delayed, but it's because they have an elderly population that wasn't very well vaccinated," he said.
US officials don't expect BA.2 to hit here as hard as it has in Hong Kong.
That's because the city has followed a zero-tolerance covid strategy.
That policy has kept cases and deaths low so far, making it a model for Covid control.
But omicron and the BA.2 subvariant have overwhelmed those defenses and started infecting a population with little previous exposure to the virus.
Hong Kong also relied on a slightly different mix of vaccines than the United States and Europe, including vaccines from Sinovac made in China and Pfizer's Comirnaty.
Health officials are looking for clues in the UK about how the BA.2 subvariant may behave in the US, but they are not analogous in every way;
in particular, the UK is more vaccinated.
Overall in the UK, 82% of adults have received a third dose of a covid-19 vaccine, something that is crucial in preventing omicron infections and hospitalizations due to how highly 'immune erosive' they are these variants, says Shaman.
In America, that number is just 36%.
Among Americans age 65 and older who are eligible for a booster, CDC data shows that 1 in 3 have not opted in for a third dose, leaving an estimated 15 million older Americans without that critical extra protection .
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What distinguishes the covid-19 BA.2 subvariant from the omicron?
0:54
Vaccine protection decreases over time
Recent studies show that the timing of vaccination is also important.
Data compiled by the UK Health Security Agency shows that the efficacy of the omicron vaccine dropped to 10% for infections, 35% for hospitalizations and 70% for deaths six months or more after vaccination. second dose.
Reinforcements restored much of that protection, but their benefits have also faded.
Four to six months after a third dose, boosters were approximately 40% to 50% effective in preventing infections with omicron and 75% to 85% effective in preventing hospitalizations, for all adults.
In the UK, around two-thirds of older people have received a second, third or fourth dose of a Covid-19 vaccine in the last five months, but only around half of older people in the US are within five months of your second or third dose vaccine.
Comparing antibody protection from a previous covid-19 infection or vaccination, the UK comes out ahead again.
By the end of February, 98% of adults in the UK had tested positive for Covid-19 antibodies, according to the National Office for Statistics.
In the United States, the CDC estimates that 43% of Americans have antibodies from a past infection to fight Covid-19.
However, older people are the least likely to have this protection, with only 23% of adults over the age of 65 testing positive for antibodies from a previous infection.
"I still think it's a potential cause for concern that we may still see a higher case fatality rate and more Covid hospitalizations in the US than the UK due to differences in underlying immunity," Kissler told CNN.
So while a wave of the BA.2 subvariant in the US may not be as dire as it is for Hong Kong, it may not be the same experience the UK is having either.
"What we see happening in the UK is going to be perhaps a better story than we should expect here," said Keri Althoff, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.
With perhaps a critical few weeks to prepare, Kissler and Shaman say vaccinations and boosters for seniors are an important place to start.
"Every extra layer of protection that we get helps, so I would highly recommend, especially someone older who hasn't gotten vaccinated yet, to get it, because it really can go a long way toward giving you the robust, long-lasting immunity that you need and wants," Kissler said.
"This is definitely the time."
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