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The deployment of a third dose of the Covid vaccine has sparked debate on ethical and political grounds, as a large portion of the human population has yet to receive any vaccine. But the case for science-based boosters is building.
The reason is delta. The most infectious variant of the coronavirus to emerge so far is in a race with the human immune system, and there is growing evidence that delta is winning – at least initially. Fully vaccinated individuals infected with the variant have levels of the virus in the upper respiratory tract as high as those who lack immunity, a large study from the UK showed last week.
This suggests that people with breakthrough delta-induced infections may also be able to transmit the virus, frustrating efforts to curb the Covid pandemic. Falling antibody levels in some highly vaccinated populations like Israel have prompted calls for boosters to ease further waves of hospitalizations.
“Science is the job of boosters, and they will definitely help,” said Shane Crotty, virologist and professor at the Center for Infectious Disease and Vaccine Research at the La Jolla Institute of Immunology in California.
In fully vaccinated, healthy adults, booster injections from Moderna Inc. as well as Pfizer Inc. and its partner BioNTech SE rebound antibodies to peak levels, if not well beyond, Crotty said in a Zoom interview on Friday. These antibodies are also likely to be more durable and able to fight a wider range of strains of SARS-CoV-2, he said.
This is particularly useful in the fight against the delta. Researchers in China have found that the strain is detectable in patients four days after catching the virus – two days earlier than previously observed – indicating that the strain makes people infectious sooner.
People infected with delta also had significantly higher amounts of the virus in their upper respiratory tract compared to infections caused by other strains.
“It’s inherently harder to stop with antibodies because there are more of them and it’s a more difficult challenge for the immune system,” Crotty said.
Raising antibody levels with an extra dose of vaccine can allow the immune system to quickly block the delta as it enters the nose and throat, preventing the coronavirus from not only infecting cells and causing disease, but also to prevent it from spreading, he said. A slower antibody response, on the other hand, can increase infectivity and worsen symptoms.
“It’s a race between the virus and your immune system,” Crotty said. The faster the virus replicates, the less time the antibodies have to block an infection.
Yet even when a delayed antibody response leads to infection, the immunity generated by vaccination or natural infection is usually sufficient to prevent it from causing serious illness in an otherwise healthy person, he said. he declares.
Most pediatric vaccine schedules are given as three injections, Crotty said. “A lot of it is about the mechanisms of immune memory generation – it often takes three exposures to get it,” he said.
Delta Concern
Three studies published by the US Centers for Disease Control and Prevention last week showed that although delta may accelerate a decline in the effectiveness of Moderna and Pfizer-BioNTech shots in preventing SARS-CoV-2 infections, including among nursing home residents, vaccines remained a reliable shield against hospitalization over a six-month period.
Delta can cause groundbreaking infections and diseases in people with lower antibody levels, said Andrew Pekosz, professor of molecular microbiology at the Bloomberg School of Public Health at Johns Hopkins University in Baltimore.
“The good news is that the vaccine’s effectiveness against serious illness is still being maintained,” he said. In addition, fully immune people infected with delta may be infectious for a shorter period compared to people without immunity, reducing their propensity to transmit the virus, Pekosz said.
It is possible that the infectivity of those vaccinated could be further reduced with third injections using a different inoculation, administration via a nasal spray, or using smaller amounts of the same vaccine, he said.
“We are 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 at Minneapolis. “Our challenge will be how to continue using these vaccines to maximize the most important aspect of public health: infection prevention. “
Limiting the spread is also key to preventing the emergence of variants that are even more dangerous than the Delta, he said.
Equity of vaccines
This goal must be pursued globally with fairness and solidarity, said Osterholm.
“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 obviously critical.
In the United States, unvaccinated people are now overwhelming medical facilities in large areas of the country, Johns Hopkins’ Pekosz said. “Vaccination would prevent people from going to the hospital, and it is the most important thing that vaccinations can do.”
Crotty de La Jolla estimates that more than 90% of the transmission of SARS-CoV-2 comes from unvaccinated people. “In the United States, we have so many,” he said.
“If you have two doses of the vaccine, it’s much more helpful for those two doses to be given to an unvaccinated person than to people receiving boosters if you mean transmission,” Crotty said. “But if you can’t convince people to get these damn things and someone else wants them now, will it really help the other person?” Yes, that will help them.
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