SAN FRANCISCO, Calif. (KRON) — The latest COVID surge has brought up new confusion and questions about what is happening with the virus in the Bay Area.

KRON4 asked UC San Francisco infectious disease expert Dr. Peter Chin-Hong commonly asked COVID questions in the “days of Delta.”

Why do breakthrough infections happen to fully vaccinated people? Why isn’t the COVID vaccine 100% effective?

DR. CHIN-HONG: “The way I think about breakthrough infection is … the vaccine is like an umbrella. It’s actually an amazing umbrella, it’s like the super expensive umbrellas. And you’re in the rain and it really keeps you dry. But when the rain is heavy, like in a thunderstorm, you can’t keep completely dry. So that’s why you get breakthrough infections with Delta. There is so much virus produced you can’t dodge all of (it).”

DR. CHIN-HONG: “But you are protected from severe disease and death. More than 99% of deaths in the U.S. are among unvaccinated folks. That means there is a super small chance. Nothing in life is 100% guaranteed. In the realm of healthcare outcomes, less than 1% is an amazing risk reduction.”

Whatever happened to herd immunity? Did San Francisco achieve herd immunity like some top medical experts were saying when summer started?

Medical experts used to loosely classify “herd immunity” as 70 percent of a population being fully vaccinated. But the Delta variant changed the rules, and now we need 85% to achieve herd immunity.

DR. CHIN-HONG: “A month ago we thought we were at herd immunity. And we probably were when the days were just Alpha and the regular Wuhan strains. But now in the ‘days of Delta’ all the rules have changed. Delta is so infectious that you need to have a lot more people immune before we can keep the infection (rates) down to a manageable level. That means we need to vaccinate a higher proportion of the population than before.”

DR. CHIN-HONG: “At the time Delta came in, we were probably in the 70% range of fully vaccinated individuals. But with Delta, most people believe we need to get at least 85% of fully vaccinated individuals, including kids.”

Do I need a booster shot?

DR. CHIN-HONG: Thanks to vaccines, “We are still having really high protection from serious disease, hospitalizations, and deaths (from the vaccine). So the CDC and federal government decided not to formally recommend a booster yet for the general population.”

When they are recommended, who will need booster shots?

DR. CHIN-HONG: “The boosters, as it suggests, boosts an immune response to protect you further. For certain populations, like immune-compromised individuals who have received transplants, or have a compromised immune system because of medications they take … the booster is needed to make sure they get some (immune) response. That will probably be the first group recommended. The other population that we may see some recommendations for is for the older individuals. In the future, antibodies (may) wane enough that it sends them to the hospital. Whatever happens, you will get very firm guidance from the CDC when, and if, the booster shot is approved in the United States.”