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Task Force’s Koopman discusses sharp drop in Delta Variant

By PAULETTE LeBLANC - | Oct 13, 2021


The basic reproduction number of the Delta Variant could be as high as 10 in certain populations, Dr. James Koopman said, at the Oct. 4 meeting of the Pine Island Task Force. The basic reproduction number is not just a characteristic of the virus, it’s also a characteristic of the populations, he explained.

“That will give you a sharp rise that carries over for a bit of time. There’s a lot of momentum from all the cases that have been generated during that sharp rise, after you reach the point where each person is only generating one other case,” Koopman said.

He went on to say just about the time we began to relax from the original outbreak of COVID-19, the Delta Variant made its way through the U.S. causing another high level of infection. Currently, we are infecting a fraction of what we were, which explains the sharp fall in numbers of cases. At the end of the fall in cases, he said, the infection still will not be eradicated; it will simply become a low level of transmission. As population immunity wanes from a previous infection or vaccination, new susceptibility will be pushed to the forefront. We will never reach herd immunity eliminating transmission, he said.

In order to experience a similar spike to the Delta Variant, there would have to be either waning immunity or another new variant, which could be produced if there are enough people who have been previously infected.

“One of the big differences between the vaccine and the natural infection, although on average the natural infection may have a little more immunity than the vaccine, is a tremendous variability from person to person in the immunity stimulated by natural infection. You still have a lot of people left at a high level of susceptibility. The second factor is the occurrence of new mutations that lead to new variants that allow the infection to take off again, because they help it escape the immunity that was previously stimulated. Waning immunity and escape mutations are the thing that bring it back up,” Koopman said.

It is unclear whether vaccines will ultimately prevent infection from new variants better than natural immunity, he said. One characteristic of the natural infection is that some people seem to get good immunity that could protect very well against variants. Conversely, there are those who will experience very low immunity, whereas the vaccines remain steady and uniform in their protection against COVID-19.

“You still have variability–some people higher than other people, but the overall amount of variation is greater after natural infection than after the vaccine,” Koopman said.