Delta can cause breakthrough infections and illness in people with lower antibody levels.
The rollout of a third dose of Covid vaccine has sparked debate on ethical and political grounds as much of the human population has not yet received any vaccinations. But the case for boosters on scientific grounds is on the rise.
The reason is delta. The most contagious coronavirus variant yet to emerge is in a race with the human immune system, and there is mounting evidence that delta is winning — at least initially. Fully vaccinated individuals infected with the variant have peak virus levels in the upper respiratory tract that are just as high as those without immunity, a large study from the UK showed last week.
That suggests that people with delta-induced breakthrough infections may also be able to transmit the virus, which frustrates efforts to curb the Covid pandemic. Declining antibody levels in some highly vaccinated populations like Israel have led to calls to offer boosters to dampen new waves of hospitalizations.
“The science is that 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 ensure antibodies return to peak levels, if not far 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.
This is especially useful when combating delta. Researchers in China found that the strain is detectable in patients four days after picking up the virus — two days earlier than previously observed — indicating that the strain makes individuals more contagious.
People infected with delta were also found to have significantly higher amounts of virus in their upper respiratory tract compared to infections caused by other strains.
“It’s inherently harder to stop antibodies because there’s more of it and it’s more of a challenge to the immune system,” Crotty said.
Boosting antibody levels with an extra dose of vaccine could allow the immune system to quickly block delta on arrival in the nose and throat, preventing the coronavirus from not only infecting cells and causing disease but also preventing it from spreading, said he. In contrast, a slower antibody response can increase infectivity and worsen symptoms.
“It’s a race between the virus and your immune system,” Crotty said. The faster the virus multiplies, the less time antibodies have to block an infection.
But even when a delayed antibody response leads to infection, immunity generated by vaccination or a natural infection is usually enough to prevent it from causing serious illness in an otherwise healthy person, he said.
Most pediatric vaccine regimens are administered over three shots, Crotty said. “A lot of it is about the mechanics of immune memory generation — that it often takes three exposures to get that,” he said.
Delta care
Three studies published last week by the U.S. Centers for Disease Control and Prevention found that while delta showed a decrease in the effectiveness of the Moderna and Pfizer-BioNTech shots in preventing SARS-CoV-2 infections, also among nursing home residents, the vaccines remained a reliable shield against hospitalization for six months.
Delta can cause breakthrough infections and disease in people with lower antibody levels, said Andrew Pekosz, a professor of molecular microbiology at Johns Hopkins University Bloomberg School of Public Health in Baltimore. “The good news is that the vaccine’s efficacy against serious diseases is still being maintained,” he said.
Virus particles from delta-infected individuals are much less likely to be contagious when emitted from fully vaccinated people, researchers in the Netherlands showed in a study released Saturday before publication. Immunized individuals are also contagious for a shorter period of time compared to those without immunity, reducing the chances of further transmission, a study from Singapore found last month.
It is possible that the infectivity of vaccinated individuals could be further reduced with a third injection of a different inoculation, administration via a nasal spray and even smaller amounts of the same vaccine, Pekosz said.
“We’re still working on how best to use these vaccines for maximum protection,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota in Minneapolis. “Our challenge will be how we can continue to use these vaccines to maximize the most important aspect of public health: preventing infections.”
Reducing the spread is also key to preventing variants that are even more dangerous than delta, he said.
Vaccine Stocks
That goal must be pursued globally with justice and solidarity, Osterholm said.
“It’s a global epidemic that will continue until you take care of it around the world,” he said. “From a humanitarian point of view, this is of course critical.”
In the US, unvaccinated people are now flooding medical facilities across much of the country, Johns Hopkins’ Pekosz said. “Vaccination would keep people out of the hospital, and that’s the most important thing vaccinations can do.”
La Jolla’s Crotty estimates that more than 90% of SARS-CoV-2 transmission comes from unvaccinated people. “In the US we have so many,” he said.
“If you have two doses of vaccine, it’s much more valuable that those two doses go to an unvaccinated person than people getting boosters if you want to talk about transmission,” Crotty said. “But if you can’t convince the people to get those damn things and someone else wants them now, will it really help the other person? Yeah, it’ll help them.”
–With help from Lisa Beyer.
(Except for the headline, this story has not been edited by NewsMadura staff and has been published from a syndicated feed.)
