The roll out of a third dose of Covid vaccine has sparked debate on ethical and political grounds, since a large swath of the human population is yet to receive any inoculation. But the case for boosters on scientific grounds is building.

The reason is delta. The most-infectious coronavirus variant to emerge so far is in a race with the human immune system, and there’s mounting evidence that delta is winning -- at least initially. Fully vaccinated individuals infected with the variant have peak virus levels in the upper airways as high as those lacking immunity, a large study from the U.K. showed last week.

That suggests people with delta-induced breakthrough infections also may be capable of transmitting the virus, frustrating efforts to curb the Covid pandemic. Waning antibody levels in some highly vaccinated populations such as Israel have prompted calls to offer boosters to blunt fresh waves of hospitalizations.

“The science is the boosters work, and they will definitely help,” said Shane Crotty, a virologist and professor at the La Jolla Institute for Immunology’s Center for Infectious Disease and Vaccine Research in California.

In fully vaccinated, healthy adults, booster shots from Moderna Inc. as well as Pfizer Inc. and its partner BioNTech SE cause antibodies to rebound to peak levels, if not well beyond, Crotty said in a Zoom interview Friday. Those antibodies are also likely to be more durable and adept at fighting a wider range of SARS-CoV-2 strains, he said.

That’s especially helpful in fighting delta. Researchers in China found the strain is detectable in patients four days after picking up the virus -- two days earlier than was previously observed -- indicating the strain makes individuals infectious sooner.

People infected with delta were also found to have significantly higher amounts of virus in their upper airways compared with infections caused by other strains.

“It’s inherently tougher to stop with antibodies because there’s more of it and it’s a tougher challenge for the immune system,” Crotty said.

Bolstering antibody levels with an extra dose of vaccine may enable the immune system to swiftly block delta on its arrival in the nose and throat, preventing the coronavirus from not only infecting cells and causing illness, but also stopping it from spreading, he said. A slower antibody response, on the other hand, may increase infectiousness and worsen symptoms.

“It’s a race between the virus and your immune system,” Crotty said. The faster the virus replicates, the less time antibodies have to block an infection.

Still, even when a delayed antibody response does lead to infection, immunity generated by either vaccination or a natural infection is usually enough to prevent it causing severe illness in an otherwise healthy person, he said.

Most pediatric vaccine regimens are administered over three shots, Crotty said. “A lot of that is about the mechanics of immune memory generation - that it’s frequently taken three exposures to get that,” he said.

Three studies published by the U.S. Centers for Disease Control and Prevention last week showed that although delta may be hastening a decline in the effectiveness of the Moderna and Pfizer-BioNTech shots at preventing SARS-CoV-2 infections, including among nursing home residents, the vaccines remained a reliable shield against hospitalization over a six-month period.

Delta can cause breakthrough infections and illness in people with lower antibody levels, said Andrew Pekosz, a professor of molecular microbiology at the Johns Hopkins University Bloomberg School of Public Health in Baltimore. “The good news is that vaccine efficacy against severe disease is still maintained,” he said.


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