New cases, hospitalizations and deaths are declining rapidly, and the supply of available vaccine is increasing.
The country may be well on its way to herd immunity, the point at which enough people are protected from a disease that it cannot spread through the population.
But it can take months to get there, and no one expects it to feel like an overnight return to our lives before the pandemic.
According to the latest federal data, more than 66 million shots were fired, with nearly 8% of the U.S. population fully vaccinated. Promises from manufacturers indicate that the US should have enough vaccine to cover all by June. More than a quarter of the population may have a natural immunity after previous infection – and this can be much higher than official counts show.
However, some new varieties threaten advances, potentially reducing vaccine protection and surrounding some natural immunity. Vaccine hesitation can also cause limitations.
Their answers are edited below for length and clarity.
There is still so much to learn about Covid-19. But if you had to put a number on it, what level of population protection is needed to achieve herd immunity against Covid-19?
Casadevall: I’m in the 65-80% range. We all expect that this virus will not differ from other viruses and that we will reach a point where enough people are immune so that the virus can no longer jump. It reaches a point that there are so few hosts, so few people that it can jump, that the epidemic crashes. The number of cases is the threat, and in the downward curve we see that the number of people he can jump to is declining.
Speaker: Community immunity is a continuum. As more immunity builds up, the disease becomes easier to control. At present, controls in place – such as masking and limiting capacity – provide a degree of protection, which contributes to the decline in business. But getting to the magic number where you can drop most of the measures and not have to worry about big outbreaks – there will still be – is between two thirds and 80%. But additional community immunity offers real benefits, even if it is on that threshold.
Malaty Rivera: We really need to vaccinate at least 70% of the population. This seems likely by the end of the year if there are no significant bottlenecks in production or delivery.
Mehta: I have seen estimates of up to 65% and up to 95%. What I really want is to make sure we reach at least 75%.
Murray: Respiratory viruses are very seasonal, so the level of immunity in summer is much lower than in winter. Covid is less dramatically seasonal than other respiratory viruses, but as we have learned, it still is. I would give a summer herd immunity threshold of 65% and for the winter 85%.
People who have already had Covid-19 may have a natural immunity after infection. Can we count those people for herd immunity?
Casadevall: Yes. The number of people known to have reinfection is very low. There have certainly been some cases, and these have been documented. But despite all this virus still circulating, people are not getting sick again, and that’s really encouraging to me.
Speaker: In the short term, over the next six months, I will count everyone who is infected and most people who have been vaccinated as immune. There are a few cases of re-infection, but it is mostly among people who had a fairly mild illness during the first infection.
Malaty Rivera: No. Herd immunity can only be discussed in the context of mass vaccination. I am on the more conservative side and very much hesitate to claim that natural immunity causes a significant difference in these numbers.
Mehta: Can be. The US has had many, many infections, but it has spread over time. It is difficult to know exactly how many people are infected, and it appears that some people lose their immunity some time after infection. The aim should be to protect the population through vaccination.
Murray: I would normally say yes, but some evidence surrounding the variant first discovered in South Africa casts doubt on it. Anything we know about the weakening natural immunity is purely speculative.
New variants could threaten the protection levels, both in terms of vaccine efficacy and possible reinfection. How much does this threat cut in terms of progress with herd immunity?
Casadevall: Variants are the biggest threat on the horizon. They have the potential to derail things, but I emphasize potential. We know variants are there, but there are still very few cases of documented reinfection. What suggests to me is that variants do not evade immune defense. The most encouraging thing for me is how fast the curve decreases. It gives me hope that we are going to crash the curve before variants become a threat.
Speaker: There are some variants that seem to escape the immunity and change the equation. In a perfectly 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 reduce the spread. But with some variants, the average person can spread the disease to about five people, so you need to have four people to be immune to reduce the spread.
Malaty Rivera: The conversation about variants should really be about preventing infection. If we keep the transmission low, we can keep the variants low. As the vaccines begin, it is time to buy time, one of the many reasons why we still wear masks and continue to practice mitigation strategies.
Mehta: We think that vaccines will provide good immunity for most variants, but there are some – especially the variant that was first identified in South Africa – that seem to have the ability to circumvent them and that could change our level of protection. That is why it is so important to vaccinate as soon as possible. The faster we reach higher levels of protection in our community, the less likely it is that new varieties will spread and develop.
Murray: There is not a very clear path to herd immunity if these variants spread, and it is a very troublesome undertaking trying to predict at what rate they will spread. But things are coming down fast, maybe even more than we expected.
Some surveys still show a significant amount of hesitation against vaccines. How will this affect the path to herd immunity?
Casadevall: The number of cases is the threat, and much depends on the vaccine intake by next year. The more cases you have, the more virus replication and the greater the likelihood of someone getting infected. If we continue as we are and the curve collapses, the likelihood of a bad scenario is reduced.
Malaty Rivera: Anti-wax people are a harsh minority. They do not represent a large group of people in this country, and I do not think they will achieve a huge victory when it comes to things like herd immunity. Usually the obstacle that people do not take a vaccine is a concept, but we have achieved great victories with campaigns to address it.
Mehta: It is really understandable why some people in our communities may be hesitant about the vaccine. What we need to do is to continue to take good care of them and to be role models, not only by sharing knowledge and taking the vaccine ourselves, but also to take precautions such as wearing a mask and to practice good hand hygiene.
Murray: If we can move from 25% of the population that is not vaccinated, to 10%, it really increases the immunity of the herd. Even if the variants are not as bad as we fear, they will still be fairly close and it will make a big difference to vaccinate over the tipping point.
What is the conclusion? When can we return to normal?
Casadevall: No one in this world can tell you what percentage of protection is needed or on what date it will take place. My gut is positive, and I do think 2021 is going to be a better year than 2020. Assuming we do not have a variant that forces new closures, I think the second half of this year will look different. We may lift restrictions in the fall, or perhaps continue a little earlier than the rapid decline and stay down the curve.
Speaker: When we come to very, very low numbers of new things, it is the sign that things can be good. If we put the controls back, we must do so slowly and deliberately, noting any revival. Some of this comes down to the decision that is normal, because the fact is that we have to accept the fact that we will live with this virus forever. It will come back again and again like the flu, but that does not mean it will always be the plague.
Malaty Rivera: Vaccinating 70% of the population would be an absolute changer for us in terms of returning to what we could do similar to days before the Covid. I am optimistic that this autumn will look very different and hope that it means things like travel and mask-free sociability with loved ones. We’re on our way to get there if we keep the trends.
Mehta: Thanksgiving is my favorite pastime when everyone in the family gets together at my mom’s house. My hope – personally and professionally – is that it takes place before Thanksgiving.
Murray: I really think it will be a quiet summer. But whether it will come back again is an open question, and we will not know the answer until December. Two powerful forces – seasonality and the extent of the vaccination – together are going to drop things pretty steadily, but we’ll have to wait and see.