A new year is upon us, and with it, more discoveries as scientists race to fully understand potential threats the Omicron variant poses. Dr. Monica Gandhi, infectious disease specialist and professor of medicine at the University of California, San Francisco, sheds light on what is known about Omicron's transmissibility and severity and how current COVID-19 vaccines fare at preventing severe illness from the variant.
MONICA GANDHI: So what we know now is that Omicron is the most transmissible variant of all, probably four times more transmissible than Delta, according to a Japanese study. And that's quite obvious, actually. It's really taken over almost everywhere, and it is definitively the most transmissible variant.
Two kind of ways to distinguish this. One is that we are in a position in December 2021, January 2022, of having much higher rates of immunity in the world, whether it's from vaccines like here in this country-- 75% one dose, 62% fully vaccinated-- or whether it's from natural immunity. So there was a recent seroprevalence study in South Africa that showed 75.6% of adults had immunity, basically, and it was 25% vaccination rate, so 50% likely for natural immunity.
So in this position of being in January 2022, having so much more immunity in the world, we are seeing much more mild disease from Omicron. And that could have been-- happened anyway with any variant, but then the variant itself, six studies total show us that Omicron doesn't infect lung cells very well at all, especially compared to all the other variants.
And so it's likely inherently milder as well, which is why when we're looking at hospitalizations, even if people who do get hospitalized, shorter stays-- three versus eight days, according to a South Africa stay-- less oxygen requirements. Usually 74% oxygen requirements, and here was something like 20% in the South Africa study. Again, different populations. So less ill even if you are unvaccinated.
When we even talk about it, we have to kind of divide our immune system always into antibodies, which are short-term produced to fight the virus, B cells, which are those deeper cells to make more antibodies in the future, and then T cells, which help us fight severe disease. We now have six studies that are showing us that T cells are working just fine. T cells from two vaccines working just fine against the Omicron variant. It covers-- there's enough T cells that cover the spike protein that 32 mutations aren't phasing your T cells. So six studies there.
However, in terms of antibodies, it-- absolutely, it looks like you need higher levels of antibodies to protect yourself from even that mild symptomatic breakthrough, that mild disease, because antibodies are working most up in these upper airways. So three shots help restore protection just in terms of the antibodies. It only likely lasts about 10 weeks, according to UK data, that third shot and their antibodies, but your T cells are still there, your B cells are still there to protect against severe disease.
It is more of an upper respiratory infection, with it not looking like it infects lung cells well and needing less of the oxygen when you go in the hospital. And children have often been susceptible with little airways to upper airway infections like RSV, bronchiolitis, parainfluenza, influenza. So children are both going in for incidental or they're going in for Omicron.
The South Africa data did not show us that it was more severe, Omicron, in children than in Delta. And in fact, the Children's Hospital of Pennsylvania put out a report two days before schools were to open just this last weekend and said they don't think schools should close for Omicron because it definitely looks milder in children than the previous variant. But when you have so much of an infection around, and you are compromised in other ways, you could get hospitalized. And you should for us to watch you, but the hospital stays are shorter.
It is a little harder with Omicron because the other infections actually led to olfactory kind of smell and taste abnormalities that we all heard about, and it doesn't look like Omicron does that. So it really does look like an upper respiratory tract infection with the typical symptoms of stuffy nose, headache, and sometimes fever and just feeling like you have a bad cold. Anecdotally, I would say that the Delta breakthrough seemed more severe than the Omicron breakthroughs, meaning they seem more mild than even the breakthrough infections that fully vaccinated people have with Delta.
But it's very hard to distinguish. It's just that we have so many tests for Omicron that we can test for Omicron. But again-- for COVID. But again, we're not doing-- in all these testing sites, for example, we're not testing for other concomitant viruses that could be in there like RSV.
When a virus is so transmissible, it is absolutely kind of getting everywhere. It can be in your nose. And our tests are very imperfect. A PCR test can just pick up a very tiny bit of virus that flew into your nose. Doesn't even mean that you're infectious or sick.
So yes, it can pick up asymptomatic Omicron very readily. And then rapid antigen tests, they're good for infectiousness, but you still may be non-infectious after a couple of days, and your rapid antigen will still be positive. So you can almost expect that if you really do mass asymptomatic screening, there would be a lot of Omicron in noses.
But remember, we never do mass asymptomatic screening for any other virus. That could be true of RSV season, parainfluenza, influenza season, adenovirus, other coronavirus seasons. It's just that we're picking up a lot because we do a lot of testing for COVID.
I do not think they should. I know that's been controversial by the CDC, and a lot of people are calling for them to have a test at 5 days, but there was a study from Italy, from the Annals of Internal Medicine, that showed that health care workers who were asymptomatic and tested positive on one day-- and this was during staffing shortages with Delta breakthroughs-- they test-- all 33 that were asymptomatic and tested positive one day were negative the next day and were negative the third day.
If you're asymptomatic, you aren't getting sick, and you likely have a very low viral load, and it goes away the next day. So doing that requirement of testing at an asymptomatic person is expensive. It's hard to find tests. And again, with such a prevalent variant, the practicalities of having people leave isolation in five days, that's a very good reason. And I thought the CDC was wise in what they did.