When will the US reach herd immunity and what will it look like?

A year into the Covid-19 pandemic, it appears that trends have finally shifted in a positive direction.

New cases, hospitalizations and deaths are dropping rapidly, and the supply of available vaccine is growing.

The country could be well on its way to herd immunity, the point at which enough people are protected against a disease that it cannot spread through the population.

But it may take months to get there, and nobody expects it will feel like an overnight return to our lives before the pandemic.

More than 66 million shots have been administered, according to the latest federal data, with nearly 8% of the US population fully vaccinated. Promises from manufacturers indicate that the US should have enough vaccine supply to cover everyone by June. More than a quarter of the population may already have natural immunity after previous infection — and that number may be much higher than official counts show.

However, some new variants threaten progress, potentially lessening protection offered by vaccines and skirting some degree of natural immunity. Vaccine hesitancy may also create some limitations.

To understand how these factors may play into the future timeline of the pandemic, CNN spoke with five experts: Dr. Arturo Casadevall, chair of molecular microbiology and immunology at Johns Hopkins University; Justin Lessler, associate professor of epidemiology at Johns Hopkins University; Jessica Malaty Rivera, science communications lead at the COVID Tracking Project; Dr. Aneesh Mehta, of the Emory Vaccine Center; and Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation.

Their responses have been edited below for length and clarity.

There’s still so much to learn about Covid-19. But if you had to put a number on it, what level of population protection is required to reach herd immunity against Covid-19?

Casadevall: I am in the 65-80% range. We all expect that this virus is not different from other viruses and that we will reach a point that a sufficient number of people are immune so that the virus cannot jump any more. It reaches a point that there are so few hosts, so few people that it can jump to, that the epidemic crashes. The number of cases is the threat, and in the declining curve, we see that the number of people it can jump to is dropping.

Lessler: Community immunity is a continuum. As more immunity builds, the disease becomes easier to control. Right now, control measures in place — such as masking and limiting capacity — are offering some protection, which is contributing to the drop in cases. But to get to the magic number where you can drop most of those measures and not worry about large outbreaks — there will still be some — is somewhere between two-thirds and 80%. But additional community immunity gives real benefits even when right at that threshold.

Malaty Rivera: We really need to be vaccinating at least 70% of the population. That seems potentially possible by the end of the year if there are no significant bottlenecks in production or delivery.

Mehta: I’ve seen estimates as low as 65% and as high as 95%. What I’d really like is to make sure we get to at least 75%.

Murray: Respiratory viruses are very seasonal, so the level of immunity required in the summer is much lower than in winter. Covid is less dramatically seasonal than other respiratory viruses, but as we’ve learned, it still is. I would give a summer herd immunity threshold of 65% and for winter 85%.

Individuals who have already had Covid-19 may have some natural immunity after infection. Can we count those people toward herd immunity?

Casadevall: Yes. The number of people known to have reinfection is very low. There have definitely been some cases, and they’ve been documented. But despite all this virus still floating around, people are not getting sick again, and to me that’s really encouraging

Lessler: In the short term, over the next six months or so, I would count everybody who’s been infected and most people who have been vaccinated as immune. There are some cases of reinfection, but they’re mostly among people who had a pretty mild disease in the first infection.

Malaty Rivera: No. Herd immunity can only be discussed in the context of mass vaccination. I’m on the more conservative side and very hesitant to claim that natural immunity is causing a meaningful difference in these numbers.

Mehta: Maybe. The US has had many, many infections, but they’ve been spread out over time. It’s hard to know exactly how many people have been infected, and it appears that certain people lose their immunity some time after infection. The goal should be to protect the population through vaccination.

Murray: I normally would say yes, but some evidence around the variant first discovered in South Africa brings that into question. Also, anything we know about waning natural immunity is purely speculative.

New variants potentially threaten levels of protection, both in terms of vaccine efficacy and possible reinfection. How much does this threat cut into measures of progress to herd immunity?

Casadevall: Variants are the big threat on the horizon. They have the potential to derail things, but I stress potential. We know variants are out there, but there are still very few cases of documented reinfection. What that suggests to me is that variants are not evading immune defense. The most encouraging thing to me is how rapidly that curve is coming down. It gives me hope that we’re going to crash the curve before variants become a threat.

Lessler: There are some variants that appear to be escaping that immunity and changing the equation. In a completely susceptible population, the average person with the original strain would spread the virus to three people, so you need two people to be immune to start decreasing the spread. But with some variants, the average person may spread the disease to around five people, so you need four people to be immune to cut the spread.

Malaty Rivera: The conversation around variants really needs to be around preventing infection. If we keep transmission low, we can keep variants low. As the vaccines roll out, one of the many reasons we’re still wearing masks and continuing to practice mitigation strategies is to buy ourselves time.

Mehta: We think vaccines will provide good immunity to most variants out there, but there are some — particularly the variant first identified in South Africa — that appear to have the ability to get around that and may alter our level of protection. That’s why it’s so important to vaccinate as quickly as we can. The quicker we get to higher levels of protection in our community, the chances for new variants to spread and develop diminishes.

Murray: There isn’t a very clear route to herd immunity if those variants spread, and it’s a very tricky business trying to predict at what pace they’ll spread. But cases are coming down quickly, perhaps even more than we had expected.

Some surveys still show a significant amount of vaccine hesitancy. How will this affect the path to herd immunity?

Casadevall: The number of cases is the threat, and a lot depends on what the vaccine uptake is by next year. The more cases you have, the more virus replication and the more likelihood that a person can be infected. If we keep going the way we are and crash the curve, the likelihood of a bad scenario is reduced.

Malaty Rivera: Anti-vax people are a loud minority. They don’t represent a large group of people in this country, and I don’t think they’ll have tremendous victory when it comes to things like herd immunity. Usually, the impediment to people not taking a vaccine is understanding, but we’ve had major wins with campaigns to address this.

Mehta: It’s really understandable why some people in our communities may have hesitation around the vaccine. What we need to do is continue to do a good job caring for them and being role models, not only by sharing knowledge and taking the vaccine ourselves, but continuing to take precautionary measures such as wearing a mask and practicing good hand hygiene.

Murray: If we can move from 25% of the population not being vaccinated to 10%, that really boosts herd immunity way up. Even if variants aren’t as bad as we fear, it’s still going to be pretty close and vaccinating over the tipping point would make a huge difference.

What’s the bottom line? Generally, when can we expect a return to normal?

Casadevall: No one in this world can tell you what percent of protection is needed or what date it will happen by. My gut is positive, and I do think 2021 is going to be a better year than 2020. Assuming we don’t have a variant that forces new lockdowns, I think the second half of this year will look different. We may lift restrictions in the fall, or maybe a little earlier if the rapid decline continues and the curve stays down.

Lessler: When we get to very, very low numbers of new cases, that’s going to be the sign that things can be fine. As we step back control measures, we should do so slowly and deliberately and watch for any resurgence. Some of this comes down to deciding what is normal, because the fact of the matter is we need to accept the fact that we will be living with this virus forever. It will come back again and again like the flu, but that doesn’t mean that it will always be the scourge that it is now.

Malaty Rivera: Vaccinating 70% of the population is going to be an absolute game changer for us in terms of getting back to what we could do similar to pre-Covid days. I’m optimistic that this fall is going to look very different, and hope that means things like travel and mask-free socializing with loved ones. We’re on the road to getting there if we keep the trends down.

Mehta: Thanksgiving is my favorite holiday, when everyone in the family comes together at my mother’s house. My hope — both personally and professionally — is that it happens before Thanksgiving.

Murray: I really do think it will be a quiet summer. But whether it will come back is an open question, that we won’t know the answer to until maybe December. Two powerful forces — seasonality and the scale of vaccination — combined are going to bring things down pretty steadily, but we’ll have to wait and see.