CNN  — 

If you think herd immunity is the finish line to this pandemic, it’s time for a reality check.

Herd immunity with Covid-19 could come and go, scientists say. Or we might never reach it at all.

“There’s a lot of things that have to go our way to actually get to herd immunity,” said Dr. Christopher Murray, director of the University of Washington’s Institute for Health Metrics and Evaluation.

And with vaccine hesitancy, “I’m not sure that we are going to reach herd immunity,” CNN Medical Analyst Dr. Leana Wen said.

But don’t panic. Here’s why it’s possible to dip in and out of herd immunity, and what you can do now to maximize the chances of snuffing out Covid-19 for good:

Why herd immunity is so difficult

“Herd immunity is the point where there’s enough people that have been either infected or vaccinated that you basically can’t sustain transmission in the community,” Murray said.

“And if one case leads to less than one new infection, then eventually it peters out.”

Estimates vary on how much of the population needs to have immunity to reach that goal.

Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, has said herd immunity might be reached if 70-85% of people are immune. Board-certified internist Dr. Jorge Rodriguez estimates that number is about 85-90%.

“If we vaccinate 70% of the whole US population, including kids, and then another 15-20% have already gotten (infected), I think we’re pretty close to reaching a normalcy with the virus,” he said.