Covid-19 Delta Variant Widens Gulf Between Vaccinated and Unvaccinated

Even as many Americans celebrate the apparent waning of the pandemic, the thrum of concern over the so-called Delta variant grows steadily louder.

The variant, the most contagious version yet of the coronavirus, accounts for more than half of new infections in the United States, federal health officials reported this month. The spread of the variant has prompted a vigorous new vaccination push from the Biden administration, and federal officials are planning to send medical teams to communities facing outbreaks that now seem inevitable.

Infections, hospitalizations and deaths are rising swiftly in some states with low vaccination rates like Arkansas, Missouri, Texas and Nevada, and are beginning to show small upticks in all of the others. The curves have also begun shifting upward in New York City, and the percentage of positive tests in the city has doubled in the past few weeks to just over 1 percent.

Nationwide, the numbers remain at some of the lowest levels since the beginning of the pandemic, but are once again slowly trending upward, prompting a debate about when booster shots might be needed to protect Americans.

“I think the United States has vaccinated itself out of a national coordinated surge, even though we do expect cases pretty much everywhere,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.

“Delta is creating a huge amount of noise, but I don’t think that it’s right to be ringing a huge alarm bell.”

Still, there are likely to be isolated outbreaks in pockets of low vaccination, he and other scientists predicted. The reason is simple: The pattern of the protection against the coronavirus in the United States is wildly uneven.

Those who have been inoculated against the coronavirus have little to worry about. Reports of infections with the Delta variant among fully immunized people in Israel may have alarmed people, but virtually all of the available data indicate that the vaccines are powerfully protective against severe illness, hospitalization and death from all existing variants of the coronavirus.

Even a single dose of vaccines that require two shots seems to prevent the most severe symptoms, although it is a flimsier barrier against symptomatic illness — making it an urgent priority to give people second doses in places like Britain that opted to prioritize first doses.

Like Israel, Britain has seen Delta infections in vaccinated people, but they have mostly been among people exposed to large amounts of the virus — health care workers, taxi and bus drivers, for example — and in those who may have mounted weak immune responses because of their age or health conditions, said Dr. Muge Cevik, an infectious disease expert at the University of St. Andrews in Scotland and a scientific adviser to the British government.

In countries with low vaccination rates, however, the Delta variant has found fertile ground. In Africa, where only about 1 percent of the population is fully immunized, the variant’s prevalence has been doubling roughly every three weeks. The number of cases across the continent rose by 25 percent and deaths by 15 percent in the week ending June 27, compared with the previous week.

That meant the most vulnerable were protected first, while the most socially active part of the population — younger people — was until recently largely unprotected. Younger people were instrumental in the spread of the virus.

In England, everyone in their late teens and 20s became eligible for shots only in mid-June, two months later than in the United States, and many are still waiting for second doses. Those second doses have become all the more crucial as Delta spreads, as the variant overwhelms the first doses in some cases.

In one study published in the journal Nature last week, only about 10 percent of blood samples from people who received one dose of either the AstraZeneca or the Pfizer-BioNtech vaccines were able to neutralize the Delta variant, compared with 95 percent of those who got both doses. (Other studies suggest that a single dose is at least enough to prevent serious illness and death, however.)

More than 90 percent of people older than 55 are fully vaccinated in Britain. That has not entirely blunted the toll on hospitals following the spread of the Delta variant: Patient admissions have begun climbing as quickly as cases in recent days, a reminder that some infections still inevitably lead to severe illness. But the proportion of cases leading to hospitalizations is lower than it was in previous waves.

“The actual transmission pattern is really strongly concentrated in the unvaccinated population, which in the U.K. is almost all young people,” said Jeffrey Barrett, who directs the coronavirus sequencing initiative at the Wellcome Sanger Institute. “You get cases, but they don’t usually get very sick.”

In the United States, some states are already seeing a rise in hospitalizations. Even if those numbers remain small compared with last winter’s, they will strain hospitals in states like Oregon, already at maximum capacity as a result of other factors, like the heat wave.

“We don’t really have a huge margin for error,” said Brian O’Roak, a geneticist at Oregon Health and Science University in Portland. “If we do see a sharp rise in hospitalizations, we’re going to be back where we were during the last surge.”

In previous waves, there was a neat, linear relationship between the number of infections, hospitalizations and deaths in the United States. Fortunately, those patterns do not hold for the Delta variant, because a large proportion of people at the highest risk now have been inoculated.

The country also opened vaccinations up to all adults, and even to 12- to 17-year-olds, which may break chains of transmission more effectively than in Britain.

The AstraZeneca vaccine dispensed in Britain appears to be less effective at preventing infections with Delta than the mRNA vaccines in wider use in the United States. That, too, could give the United States an advantage over the variant.

Because of Delta’s partial ability to undermine the immune system, the rate of breakthrough infections — cases that occur despite vaccination — with the variant appears to be greater than with previous forms of the virus, except for Beta.

Many experts have worried that even mild infections raise the risk of so-called long Covid, the constellation of symptoms that can persist months after an active infection is resolved. That raised a horrifying prospect: a surge in long-term illness throughout unvaccinated regions.

But many scientists now believe that breakthrough infections are unlikely to cause the syndrome. When a vaccinated person is infected, the virus may go through a few rounds of replication, but “the immune response is so quick and so robust that it basically stops the infection in its tracks,” said Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada.

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