(CNN)It sounds confusing -- federal health officials say the Covid-19 vaccines are working well, providing more than 90% protection against severe disease and death. They're keeping people out of the hospitals.
Yet they also say studies are showing that even vaccinated people are more likely to become infected now, so they are laying plans now for providing boosters, if federal regulators give the go-ahead.
How can both be true?
It's because of a triple whammy of naturally waning immunity, a fast moving new variant, and a population that's been slow to get vaccinated in the first place.
The coronavirus vaccines, especially the Moderna and Pfizer/BioNTech vaccines, are remarkably effective -- providing upwards of 90% effectiveness against infections that cause symptoms.
But it's important to remember that vaccines do not stop the virus cold.
"Some people think that if they are vaccinated, there is some sort of force field surrounding them," said Scott Hensley, an immunologist and microbiologist at the University of Pennsylvania.
However, if virus is in the air, even vaccinated people will breathe it in. What immunity does is control what happens after that.
Antibodies matter
The first line of immunity comes in the form of antibodies. These proteins can attach to an invader like a virus, and either make it harder for it to attack cells, or completely neutralize it.
A vaccine boosts levels of these antibodies, and trains the body to produce antibodies specifically designed to stop a pathogen such as coronavirus.
Antibodies can stop viral infection quickly.
This production starts to wane over time, in no small part because the body needs to make antibodies against other invaders, and there's only so much room.
Plus, some of the new variants have evolved mutations that help them evade antibodies.
"With some variants the virus may actually be able to get into cells and replicate for a round or two," Hensley said.
That may be what's happening in the US. The US Centers for Disease Control and Prevention published two studies Wednesday that showed immunity fell among people over the summer. While the vaccines still were 90% protective against severe disease and death, the number of people getting mild or asymptomatic infections grew.
"Recent data makes clear that protection against mild and moderate disease has decreased over time. This is likely due to both waning immunity and the strength of the widespread Delta variant," US Surgeon General Dr. Vivek Murthy told a White House briefing Wednesday
One study of nursing home residents showed immunity from any kind of infection fell from 75% in March to 53% in August.
But a third dose of vaccine boosts these antibody levels tenfold, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told the briefing. "Higher levels of antibody may be required to protect against Delta," he said.
A second line of defense
There is a second line of defense involved -- the cellular response.
Viruses attach to certain cells in the body and inject their own genetic material into them, hijacking the cells' natural functions and forcing them to become virus factories.
Immune cells called T cells can recognize these hijacked cells and work together to kill them before they produce more virus. B cells set up a longer-lasting production of antibodies, and can help recognize and neutralize viruses, also.
This longer-term immune response is likely what keeps people out of the hospital, Hensley said. B cells and T cells cannot prevent infection, but they nip it in the bud, before people become severely ill.
"The virus is cleared much more effectively in vaccinated individuals," Hensley said.
This is where some of the debate over the need for boosters comes in. The World Health Organization and some infectious diseases experts note that the vaccines are still doing their most important job—preventing severe disease and death.
"The third dose will likely do very little for further boosting the vaccine's ability to reduce hospitalizations and deaths. That's because the vaccine is already pretty darn good at that," Hensley said.
But federal health officials said while there's no indication this second level of protection has started to wane in the US, data from Israel hints that it may have begun to happen there. Because Israel vaccinated most of its population quickly, US officials are looking to surveillance there to predict what might happen in other countries.
New variants
The Delta variant now accounts for 99% of newly diagnosed infections in the US, according to CDC data. It's clearly more transmissible than earlier variants. This on its own could account for the new cases, but there's growing evidence it can bypass that first line of defense set up by antibodies.
"Vaccine effectiveness is generally decreased against the Delta variant," CDC Director Dr. Rochelle Walensky told the White House briefing.
To support this idea, Walensky cited so-called cohort studies -- which are studies that follow the same group of people over time.
One, covering 4,000 healthcare workers and other first responders, found vaccine effectiveness against either symptomatic or asymptomatic infection fell from 92% prior to the arrival of Delta to 64% once it became widespread.
What made CDC think Delta was responsible was this: it didn't matter when these volunteers were vaccinated -- it was the arrival of Delta that made them more likely to become infected.
The air we breathe
No vaccinated person would be at risk of a breakthrough infection if the virus wasn't still circulating.
Coronavirus is airborne, and as long as people are unprotected and breathing, they'll spread it.
"Even if you're vaccinated, we still all breathe the same air," Hensley said.
"Our chances of actually breathing in SARS-CoV-2, whether you are vaccinated or not, is still the same."
That's why the CDC has called for even vaccinated people to start wearing masks again indoors, when they might be exposed to spread.
"It's the only way you are going to prevent the virus from going up your nose," Hensley said.
And while no study has shown this yet, there's a growing belief among scientists that it takes a smaller dose of Delta to infect people than with previous variants. So even if there's just a small amount of virus floating in the air, if people breathe it in, they may be more likely to become infected.
And any infected person might infect someone else. Coronavirus can be spread by people who do not have symptoms.
However, vaccinated people almost certainly clear that infection more quickly. "The overwhelming majority of hospitalizations and death continue to occur among the unvaccinated," Murthy noted.
Herd immunity
So health officials, doctors and nurses across the country are clamoring for more Americans to get vaccinated and WHO is pushing for more vaccines to be distributed to the rest of the world.
So long as people are becoming infected with coronavirus and spreading it, the virus will infect some vaccinated people. And it will evolve into new forms, perhaps versions that can more easily evade the vaccines.
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That's what herd immunity is about -- when enough people are immune to infection that a virus stops circulating. This usually only happens with widespread vaccination.
The US Food and Drug Administration and the Centers for Disease Control and Prevention must make the decision to give people booster shots, but White House officials said they wanted to have a plan ready to go if and when that happens.
"We're not saying you need a booster dose right now," Walensky told CNN Wednesday evening.
"We're saying we're starting to see waning in vaccine effectiveness against moderate and mild disease and we're preparing for the month ahead because we've seen in other countries that that could portend waning for severe disease."
