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.