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When will the United States obtain herd immunity?



The number of new cases, hospitalizations and deaths is rapidly declining, and the supply of available vaccines is increasing.

The country may be moving towards herd immunization, at which point enough people are protected from diseases that cannot be spread among the population.

But it may take months to get there, and no one expects it to return to our lives overnight like before the pandemic.

According to the latest federal data, more than 66 million injections have been performed and nearly 8% of the US population has been vaccinated. The manufacturer’s commitment indicates that by June, the United States should have enough vaccine supplies to cover everyone. After previous infections, more than a quarter of the population may already have natural immunity, and this number may be much higher than the official statistics.

However, some new variants threaten progress and may reduce the protection provided by vaccines and weaken some degree of natural immunity. Vaccine hesitation may also cause some restrictions.

To understand how these factors affect the future timeline of the pandemic, CNN interviewed five experts: Dr. Arturo Casadevall, Professor of Molecular Microbiology and Immunology at Johns Hopkins University; Justin, Associate Professor of Epidemiology at Johns Hopkins University Lessler; Jessica Malaty Rivera, Head of Science Communication for the COVID Tracking Project; Dr. Aneesh Mehta of Emory Vaccine Center; and Dr. Christopher Murray, Director of the Institute for Health Metrics and Evaluation .

Their answers have been edited below to make the space clearer.

There is much more knowledge about Covid-19. However, if you have to add a number to it, what level of population protection is needed to achieve the cattle immunity against Covid-19?

Casadevall: I am in the 65-80% range. All of us hope that this virus is no different from other viruses and that we will achieve sufficient immunity so that the virus does not jump anymore. The reason for this is that there are very few hosts to which the epidemic has crashed, and very few people can jump to it. The number of cases is a threat, and in the downward curve, we see that the number of people who can jump is falling.

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Lesler: Community exemption is a continuous process. As immunity increases, diseases become easier to control. Currently, the control measures that have been taken (such as cover-up and capacity restriction) are providing some protection, which helps to reduce the number of cases. But to get a magic number, you can delete most of these measures without worrying about big outbreaks-there will still be some-between about two-thirds and 80%. However, even within this threshold, the additional community immunity will bring real benefits.

Malaty Rivera: We do need to vaccinate at least 70% of the population. If there are no obvious bottlenecks in production or delivery, then by the end of this year, this seems possible.

Mehta: The estimates I see are as low as 65% and as high as 95%. What I really want is to make sure we reach at least 75%.

Murray: Respiratory viruses are seasonal, so the level of immunity required in summer is much lower than in winter. Compared with other respiratory viruses, the seasonality of Covid is not so severe, but as far as we know, it is still the case. I set the immune threshold of the summer group to 65% and 85% in winter.

Individuals who have been infected with Covid-19 may have certain natural immunity after infection. Can we expect those people to get herd immunity?

Casadevall: Yes it is. The number of people known to have reinfection is very small. There must be some cases and they have been recorded.But even though all these viruses are still spreading everywhere, people are not regrowth, which to me is really encouraging

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Lesler: In the short term, for the next six months or so, I will treat everyone who is infected and most people who have been vaccinated as immunized. There are some cases of reinfection, but most of them are in people with fairly mild disease in the first infection.

Malaty Rivera: Not possible. Only in the context of mass vaccination can cattle immunity be discussed. I stand on the more conservative side, very hesitant to claim that natural immunity will cause meaningful differences in these numbers.

Mehta: may be. The United States is infected with many, many bacteria, but over time, they have spread. It is difficult to know exactly how many people have been infected, and it seems that some people lose immunity at some time after infection. The goal should be to protect the population through vaccination.

Murray: I usually say yes, but some evidence about the variant first discovered in South Africa casts doubt on this. In addition, everything we know about weakening natural immunity is purely speculative.

In terms of vaccine efficacy and possible reinfection, new variants may threaten the level of protection. To what extent does this threat reduce the measurement of the immunity of the herd?

Casadevall: Variants are the biggest threat to come. They have the potential to derail things, but I emphasize the potential. We know there are variants, but there are still very few recorded cases of reinfection. To me, this shows that the variant has not evaded immune defenses. For me, the most encouraging thing is how fast the curve is falling. It gave me hope that we will break the predicament before the variant becomes a threat.

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Lesler: There are some variants that seem to evade this immunity and change the equation. In a completely susceptible population, ordinary people with the original strain will spread the virus to three people, so you need two people to be immunized to start reducing transmission. But with some variants, the average person may spread the disease to about five people, so you need four people to be immune to reduce transmission.

Malaty Rivera: The discussion about variants really needs to revolve around preventing infection. If we keep the gearbox low, we can keep the variant low. With the introduction of the vaccine, one of the many reasons why we still wear masks and continue to implement mitigation strategies is to buy time for ourselves.

Mehta: We believe that vaccines can provide good immunity to most variants, but some variants (especially those first discovered in South Africa) seem to be able to overcome this situation and may change our level of protection. This is why it is so important to get vaccinated as soon as possible. The faster we get a higher level of protection from the community, the less opportunities for the spread and development of new variants.

Murray: If these variants spread, there is no clear way to gain immunity to the herd, and trying to predict at what speed they will spread is a very tricky thing. But the number of cases is rapidly decreasing, and it may even be more than we expected.

Some surveys still show a lot of vaccine hesitation. How will this affect the way cattle are immune?

Casadevall: The number of cases is a threat and depends largely on the vaccine intake next year. The more cases you have, the more the virus replicates, and the more likely a person is to be infected. If we continue to maintain the status quo and collapse the curve, then the possibility of bad things will be reduced.

Malaty Rivera: Anti-Vax people are a minority. They do not represent a large group of people in this country, and I don’t think they will achieve great victories in areas such as herd immunity. Usually, the barrier to people not getting vaccinated is understanding, but we have achieved a major victory in solving this problem.

Mehta: It is understandable that some people in our community may hesitate about vaccines. What we need to do is to continue to take care of them and become role models, not only by sharing knowledge and taking vaccines by ourselves, but also by continuing to take preventive measures, such as wearing masks and good hand hygiene.

Murray: If we can increase the proportion of the unvaccinated population from 25% to 10%, it can indeed improve the immunity of the cattle herd. Even if the variant is not as bad as we fear, it will still be very close, and vaccination at the tipping point will make a huge difference.

What is the bottom line? Generally, when can we expect to return to normal?

Casadevall: No one in this world can tell you what percentage of protection is needed or on what date it will happen. My intuition is positive, and I do think that 2021 will be better than 2020. Assuming that we have not forced the new variant of the lock-in measures, I think it will be different in the second half of this year. We may lift the restrictions in the fall, or if the rapid decline continues and the curve maintains a downward trend, it may be earlier.

Lesler: When we deal with very few new cases, this will indicate that the situation may be fine. When we back off control measures, we should do so slowly and consciously, and watch for any recovery. Part of it comes down to determining normal phenomena, because the fact of this matter is that we need to accept the fact that we will always live with this virus. It will come back again and again like the flu, but this does not mean that it will always be the current scourge.

Malaty Rivera: In terms of returning to something similar to what was done in the days before Covid, vaccinating 70% of the population would be an absolute game changer for us. I am optimistic that this autumn will look very different, and hope it means travel and maskless socializing with relatives, etc. If we keep the trend down, we are moving towards the goal.

Mehta: Thanksgiving is my favorite holiday, everyone in the family gathers at my mother’s house. My personal and professional hope is that it will happen before Thanksgiving.

Murray: I do think it will be a quiet summer. However, whether it will reappear is still an open question, and we may have to wait until December to know the answer. The combination of the two powerful forces of seasonality and the scale of vaccination will make the situation steadily decline, but we must wait and see.


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