New cases, hospitalizations and deaths are decreasing rapidly, and the supply of available vaccine is growing.
The country may be on its way to herd immunity, the point at which enough people are protected from a disease that cannot spread to the population.
But it can take months to get there, and no one expects it to look like a nightly return to our lives before the pandemic.
More than 66 million vaccines have been administered, according to the latest federal data, with almost 8% of the US population fully vaccinated. The manufacturers’ promises indicate that the United States must have enough vaccine stock to cover everyone by June. More than a quarter of the population may already have natural immunity after a previous infection – and that number could be much higher than what official counts show.
However, some new variants threaten progress, potentially decreasing the protection offered by vaccines and circumventing some degree of natural immunity. Vaccination hesitation can also create some limitations.
Your answers have been edited below for length and clarity.
There is still a lot to learn about Covid-19. But if you had to put a number on it, what level of population protection is needed to achieve collective immunity against Covid-19?
Casadevall: I’m in the 65-80% range. We all hope that this virus is no different from other viruses and that we have reached a point where enough people are immune so that the virus can no longer jump. There comes a point where there are so few hosts, so few people to jump into, that the epidemic has broken out. The number of cases is the threat and, on the decline curve, we see that the number of people it can jump to is decreasing.
Lessler: Community immunity is a continuum. As immunity increases, the disease becomes easier to control. At the moment, the control measures in place – such as masking and capacity limitation – are offering some protection, which is contributing to the reduction of cases. But getting to the magic number where you can abandon most of these measures and not worry about major outbreaks – there will still be some – is somewhere between two thirds and 80%. But additional community immunity has real benefits, even when you’re well on that threshold.
Malaty Rivera: We really need to vaccinate at least 70% of the population. This seems potentially possible by the end of the year, if there are no significant bottlenecks in production or delivery.
Mehta: I saw estimates as low as 65% and as high as 95%. What I would really like is to make sure that we reach at least 75%.
Murray: Respiratory viruses are very seasonal, so the level of immunity required in summer is much lower than in winter. Covid is less seasonally dramatic than other respiratory viruses, but, as we’ve learned, it still is. I would give a herd immunity limit in the summer of 65% and in the winter of 85%.
Individuals who have previously had Covid-19 may have some natural immunity after infection. Can we count these people for herd immunity?
Casadevall: yea. The number of people with reinfection is very low. There have definitely been some cases, and they have been documented. But even though all this virus is still circulating, people are not getting sick again, and to me this is very encouraging
Lessler: In the short term, in the next six months or so, I would count everyone who has been infected and most people who have been vaccinated as immune. There are some cases of reinfection, but most occur among people who had a very mild illness at the first infection.
Malaty Rivera: No. The herd’s immunity can only be discussed in the context of mass vaccination. I am on the more conservative side and very hesitant to say that natural immunity is making a significant difference in these numbers.
Mehta: It might be. The United States has had many infections, but they have spread over time. It is difficult to know exactly how many people have been infected and it appears that certain people lose their immunity some time after infection. The objective 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 calls this into question. In addition, everything we know about declining natural immunity is purely speculative.
New variants potentially threaten protection levels, both in terms of vaccine effectiveness and possible reinfection. How much does this threat affect measures of progress for herd immunity?
Casadevall: Variants are the big threat on the horizon. They have the potential to disrupt things, but I emphasize the potential. We know there are variants, but there are still few cases of documented reinfection. What this suggests to me is that the variants are not avoiding immune defense. What is most encouraging to me is how quickly this curve is falling. It gives me hope that we will break the curve before variants become a threat.
Lessler: There are some variants that seem to be escaping this 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 slowing the spread. But with some variants, the average person can transmit the disease to about five people, so you need four people to be immune to reduce the spread.
Malaty Rivera: The conversation about variants really needs to be around infection prevention. If we keep the transmission low, we can keep the variants low. As vaccines are launched, one of the many reasons why we are still wearing masks and continue to practice mitigation strategies is to buy time.
Mehta: We believe that vaccines will provide good immunity to most variants, but there are some – particularly the variant first identified in South Africa – that appear to have the ability to work around this and may alter 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 chances of new variants spreading and developing decrease.
Murray: There is no clear path to herd immunity if these variants spread, and it is very difficult to predict at what rate they will spread. But cases are declining rapidly, perhaps even more than we expected.
Some research even shows a significant amount of hesitation about the vaccine. How will this affect the path to herd immunity?
Casadevall: The number of cases is the threat, and much depends on how the vaccination will be in the next year. The more cases you have, the more the virus will replicate and the more likely a person will be infected. If we continue as we are and break the curve, the likelihood of a bad scenario will be reduced.
Malaty Rivera: Antivax people are a noisy minority. They don’t represent a large group of people in this country and I don’t think they will have a tremendous victory when it comes to things like herd immunity. Normally, the impediment to people not getting the vaccine is understanding, but we have had great victories with campaigns to resolve this.
Mehta: It is really understandable that some people in our communities are hesitant about the vaccine. What we need to do is continue to do a good job taking care of them and being role models, not just sharing knowledge and getting the vaccine ourselves, but continuing to take precautionary measures, like wearing a mask and practicing good hand hygiene.
Murray: If we manage to move from 25% of the unvaccinated population to 10%, it really increases the herd’s immunity. Even if the variants are not as bad as we fear, it will still be very close and vaccinating beyond the inflection point would make a big difference.
What is the end result? Generally, when can we expect a return to normal?
Casadevall: No one in this world can say what percentage of protection is needed or on what date it will happen. My instinct is positive and I think 2021 will be a better year than 2020. Assuming we don’t have a variant that forces new blocks, I think the second half of this year will be different. We can lift the restrictions on the fall, or maybe a little earlier, if the rapid fall continues and the curve remains low.
Lessler: When we get to a very low number of new cases, that will be a sign that things can be okay. As we retreat into control measures, we must do so slowly and deliberately and observe any resurgence. Part of it comes down to deciding what is normal, because the fact is that we need to accept the fact that we will live with this virus forever. It will come back again and again like the flu, but that doesn’t mean it will always be the scourge it is now.
Malaty Rivera: Vaccinating 70% of the population will be an absolute game changer for us in terms of going back to what we could do similar to pre-Covid days. I am optimistic that this fall will be very different, and I hope it means things like traveling and socializing without mask with your loved ones. We are on the right path to get there if we keep trends down.
Mehta: Thanksgiving is my favorite holiday, when everyone in the family gets together at my mom’s house. My hope – both personally and professionally – is that it will happen before Thanksgiving.
Murray: I really think it will be a peaceful summer. But whether he will return is an open question, to which we will not know the answer until perhaps December. Two powerful forces – seasonality and vaccination scale – combined will bring things down steadily, but we will have to wait and see.