“The vaccines — they’re beautiful, they work, they’re amazing,” said Frances Lund, a viral immunologist at the University of Alabama at Birmingham. “But they’re not going to give you that local immunity.”
When people are exposed to any respiratory pathogen, it may find a foothold in the mucosal lining of the nose — without causing any harm beyond that. “If you walked down the street and swabbed people, you would find people that had viruses in their mucosa who were asymptomatic,” said Dr. Michael Marks, an epidemiologist at the London School of Hygiene and Tropical Medicine. “Our immune system is mostly fighting these things off most of the time.”
But the Delta variant seems to flourish in the nose, and its abundance may explain why more people than scientists expected are experiencing breakthrough infections and cold-like symptoms.
Still, when the virus tries to snake down into the lungs, immune cells in vaccinated people ramp up and rapidly clear the infection before it wreaks much havoc. That means vaccinated people should be infected and contagious for a much shorter period of time than unvaccinated people, Dr. Lund said.
“But that doesn’t mean that in those first couple of days, when they’re infected, they can’t transmit it to somebody else,” she added.
To stop the virus right where it enters, some experts have advocated nasal spray vaccines that would prevent the invader from gaining purchase in the upper airway. “Vaccine 1.0 should prevent death and hospitalization. Vaccine 2.0 should prevent transmission,” Dr. Tal said. “We just need another iteration.”