from the last 2 posts.

What would help, [epidemiologist] Lipsitch [at the Harvard T.H. Chan School of Public Health] notes, is if public health experts knew the total number of cases worldwide, tested or otherwise, and how much protection those cases had from reinfection—two types of data that are still lacking. That leaves vaccination as the only way to reliably establish predictable levels of immunity in the population.

Populations develop immunity through a combination of vaccination and natural infections, Weitz [biologist at Georgia Tech] says, and the safe and ethical route to reach herd immunity is through vaccinations.

But if the virus keeps evolving or immunity fades over time, say in six months or a year after infection or vaccination, then as people become vulnerable to reinfection, transmission chains will start up again and the conditions of herd immunity would no longer be met. “Our group has taken to calling it ‘transient herd immunity,’” [epidemiologist] Lavine says.

“The reality is that we don’t know when herd immunity will occur. We haven’t had this disease before. We can’t say for sure what percentage of the population needs to be reached,” says Manisha Juthani, MD, a Yale Medicine infectious diseases specialist. “There is not a certain number or cut-off; it’s a gradient, meaning we’ll know we’ve reached it if we see the number of cases, hospitalizations, and deaths go down. An overall improvement of our numbers will tell us we are reaching that threshold.”

“We need to get vaccinated as soon as we can so we can prevent the spread of COVID-19, which will also prevent the emergence of variants. It’s a race between vaccination and variants.” — Ellen Foxman, Yale Medicine pathologist

Viruses like the flu...are different from measles in that they mutate over time, meaning antibodies from a previous infection won’t provide protection for long. That’s why the flu vaccine is reformulated each year to match what is expected to be the dominant strain in the coming season. Likewise, the virus that causes COVID-19 has been mutating.

[Dr. Saad Omer, director of the Yale Institute for Global Health]: "We know there is a threat of variants. For the variants in the U.S., lab evidence suggests that these vaccines will continue to work against them. But if we continue to let this pandemic run wild in places like India, there is a probability that there will eventually be a variant against which the vaccines will be less effective.”