5 reasons why COVID-19 herd immunity is difficult to achieve

5 reasons why COVID-19 herd immunity is difficult to achieve.  About 50% of Israelis have been vaccinated against COVID-19, but herd immunity is still difficult to achieve.

Even if COVID-19 vaccination is vigorously promoted, it seems that the theoretical threshold of herd immunity is still out of reach.

With the increase in the global COVID-19 vaccination rate, people have reason to ask: Is the COVID-19 virus epidemic coming to an end? Unfortunately, this is a problem full of uncertainty. The once-popular idea was that enough people will eventually gain immunity to SARS-CoV-2 to prevent most transmission-the so-called “herd immunity threshold.” However, it seems that this threshold is still far away.

This threshold is generally only reached when the vaccination rate is high. Many scientists once believed that once people started collective immunization, society would return to normal. Most estimates believe that the population who gained immunity through vaccination or past exposure to the virus needs to reach 60-70% of the total population to achieve herd immunity. But as the epidemic entered its second year, people’s thinking began to change. In February of this year, independent data scientist Youyang Gu changed the name of his popular COVID-19 prediction model from “The Road to Herd Immunity” to “The Road to Normal”. He pointed out that due to factors such as hesitation in vaccination, the emergence of new mutations, and the delay in the availability of vaccines for children, it seems impossible to reach the herd immunity threshold.

Gu is a data scientist, but his ideas are consistent with many people in the field of epidemiology. Lauren Ancel Meyers, an epidemiologist and executive director of the COVID-19 Modeling Consortium at the University of Texas at Austin, said they abandoned the idea of ​​herd immunity. Although this shift reflects the complexity and challenge of the pandemic, it does not deny the fact that vaccination is working. Although the vaccine will mean that the virus will begin to dissipate on its own, as new variants emerge and immunity to infection may weaken, we may still be fighting this threat in a few months or a year, and we must not Do not respond to the threat that may surge in the future.

The long-term prospects of the pandemic may include COVID-19 becoming a localized epidemic, just like the flu. But in the near term, scientists are considering a new normal that does not include herd immunity. Here are some of the reasons behind this mentality and their impact on next year’s pandemic.

1. It is unclear whether the vaccine can prevent transmission

The key to herd immunity is that although one person is infected, there are too few susceptible hosts around, making the virus unable to maintain the spread-those who have been vaccinated or have been infected with the virus cannot infect and spread the virus. For example, the COVID-19 vaccines developed by Moderna and Pfizer-BioNTech are extremely effective in preventing symptomatic diseases, but it is still unclear whether they can protect people from infection or prevent the virus from spreading to others. This causes problems for herd immunity.

Shweta Bansal, a mathematical biologist at Georgetown University in Washington, DC, says herd immunity only makes sense if we have a vaccine that blocks transmission. If vaccines cannot stop transmission, then the only way to gain herd immunity among the population is to vaccinate everyone. The effectiveness of vaccines to prevent transmission needs to be “quite high” to be effective, and current data are not conclusive. Although the data from Moderna and Pfizer-BioNTech seem quite encouraging, it is not clear to what extent these vaccines and other vaccines prevent people from spreading the virus. This information is critical.

The ability of a vaccine to stop transmission does not need to be 100% to be effective. Samuel Scarpino, an infectious disease scientist at Northeastern University in Boston, Massachusetts, mentioned that even if 70% of the effectiveness is “surprising” enough, there will still be a large number of viruses spreading at this time and want to interrupt the virus. The transmission chain is not easy.

2. Vaccine promotion is uneven

Matt Ferrari, an epidemiologist at Pennsylvania State University’s Center for Infectious Disease Dynamics (Pennsylvania State University’s Center for Infectious Disease Dynamics) pointed out that for various reasons, the speed and distribution of vaccine promotion are very important for herd immunity. A perfectly coordinated global action could have eliminated COVID-19, at least in theory. This is technically feasible, but in reality, we are unlikely to achieve this goal on a global scale. There are huge differences in the efficiency of vaccine promotion between countries and even within countries (Figure: Differences in vaccination rates among countries).

Israel began to vaccinate its citizens in December 2020, thanks in part to an agreement with Pfizer-BioNTech-Israel shares data with Pfizer-BioNTech in exchange for vaccine doses. Israel currently belongs to the world in terms of vaccine promotion. Lead. Dvir Aran, a biomedical data scientist at the Technion-Israel Institute of Technology in Haifa, said that during the initial stage of vaccination promotion, health workers vaccinated more than 1% of Israel’s population every day. As of mid-March, about 50% of the country’s population had been vaccinated with the two doses of vaccine needed to produce a protective effect. Aran pointed out that the problem now is that young people do not want to be vaccinated. Therefore, the local government uses things like free pizza and beer to attract these people for vaccination. At the same time, Israel’s neighbours Lebanon, Syria, Jordan, and Egypt have vaccination rates that are less than one percent of the total population.

The vaccination situation varies across the United States. Some states, such as Georgia and Utah, have less than 10% of the fully vaccinated population, while Alaska and New Mexico have more than 16% of the fully vaccinated population.

In most countries, vaccine distribution is stratified by age and is given priority to the elderly because the elderly are at the highest risk of dying from COVID-19. However, it remains to be seen when and whether a vaccine will be approved for use in children. Pfizer-BioNTech and Moderna are currently recruiting young people to participate in clinical trials of their vaccines, and Oxford-AstraZeneca and Sinovac Biotech’s vaccines are being tested on three-year-old children. But the results will not come out in a few months. Bansal pointed out that if it is impossible to vaccinate children, then more adults need to be vaccinated to achieve herd immunity (people aged 16 and over can receive the vaccine produced by Pfizer-BioNTech, but other vaccines are only allowed for 18 years old and People above are vaccinated). For example, according to the 2010 census data, the proportion of the population under the age of 18 in the United States is 24%. If most people under the age of 18 cannot be vaccinated, then 100% of people over the age of 18 must be vaccinated to achieve 76% of the population’s immunity.

Bansal said that another important issue to consider is the geographic structure of herd immunity. She pointed out that no community is an island, and the vaccination situation in the area around a community is also important for herd immunity in that community. Due to people’s actions or local policies, COVID-19 has occurred in clusters across the United States. Previous vaccination work has shown that geographically, vaccinated or not vaccinated will also tend to cluster. For example, local resistance to the measles vaccine led to a resurgence of the disease. Geographical aggregation will make the path of the immune group no longer a straight line, which essentially means that we will play a game of whack-a-mole with the COVID-19 virus epidemic. Even for a country with a high vaccination rate, such as Israel, if neighboring countries do not take the same measures, and the people of this country interact with people from other countries, the possibility of a new outbreak still exists.

3. New genetic mutations change the process of herd immunity

The current vaccine promotion plan is facing distribution obstacles, and new mutations of SARS-CoV-2 are constantly appearing. They may be more infectious and resistant to vaccines. Sara Del Valle, a mathematics and computational epidemiologist at Los Alamos National Laboratory in New Mexico, said they are racing against new mutations. The longer it takes to stop the virus from spreading, the longer it will take for these mutations to appear and spread.

What happened in Brazil was a wake-up call for the world. Research published in the journal Science shows that the slowdown in the COVID-19 outbreak in Manaus between May and October may be due to the herd immunity effect. Ester Sabino, an immunologist at the University of São Paulo in Brazil, calculated that by June 2020, more than 60% of the population in the region will be infected. According to some estimates, this should be enough to bring the population to the herd immunity threshold, but in January 2021, a large-scale recovery of cases occurred in Manaus. This surge occurred after a new mutation called P.1 appeared, indicating that the immunity generated by the previous infection was not sufficient to resist the new virus strain. Sabino pointed out that all COVID-19 cases in Manaus in January were caused by P.1. Scarpino suspects that the 60% figure may be overestimated. Even so, he believes that in the face of high levels of immunity, the COVID-19 virus will still resurrect.

Ferrari said that as herd immunity increases, there is another problem that needs to be solved. A higher immunity rate creates selective pressure, which promotes the production of virus strains that are resistant to existing vaccines. Vaccination quickly and thoroughly can prevent the creation of new mutations. But Ferrari reminded that the imbalance in vaccine promotion has once again brought challenges. Although the group has a certain degree of immunity, the virus still has some spread. Vaccines will almost inevitably induce the production of new strains, which is why the infrastructure and processes for monitoring new strains need to be established.

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In January of this year, Brazil began to distribute Sinovac Biotech’s coronavirus vaccine widely.

4. Immunity may not exist for a long time

The calculation of herd immunity generally considers two sources of individual immunity-vaccines and natural infections. Bansal pointed out that people who have been infected with SARS-CoV-2 seem to have some immunity to the virus, but how long this immunity can last is still a question. In view of the known situation of other coronaviruses and preliminary research evidence of SARS-CoV-2, infection-related immunity seems to weaken over time, so this needs to be taken into account in the calculation. Bansal reminded that although they still lack conclusive data on the decline in immunity, they know that the decline is neither 100% nor completely zero.

When calculating a population that is close to the herd immunity threshold, the modeler cannot include all infected people. They must also take into account the fact that the vaccine is not 100% effective. If infection-based immunity can only last for a few months or so, it is necessary to provide vaccines as soon as possible. It is also important to understand how long vaccine-based immunity can last, and whether a booster dose is needed over time. For these two reasons, COVID-19 may become like the flu.

5. Vaccines may change human behavior

Aran pointed out that considering the current vaccination rate, Israel is approaching the theoretical herd immunity threshold. The problem is that as more people get vaccinated, the interaction between them will increase, which changes the course of herd immunity, and this depends to a certain extent on how many people are exposed to the virus. Vaccines are not 100% effective. Imagine that a vaccine can provide 90% protection. If you only see one person before the vaccination, and now you have the vaccination, you will be unscrupulous and go to see a dozen people. It doesn’t matter whether you get the vaccination or not. Difference.

Meyers pointed out that the most challenging aspect of modeling COVID-19 is the part of sociology. So far, our understanding of human behavior has been left behind because we are living in unprecedented times and in unprecedented ways. Meyers and others try to continuously adjust their models to adapt to changes in behaviors such as wearing masks and social distancing.
Del Valle said that non-pharmacological interventions will continue to play a key role in controlling cases. The key is to break the route of transmission, limit social contact and continuous protective behaviors, such as wearing masks, can help reduce the spread of new mutant viruses when the vaccine is launched.

But it is difficult to prevent people from returning to pre-pandemic behavior. Texas and some other states in the United States have lifted the requirement to wear masks, although a significant portion of the population in these states is still unprotected. Scarpino again, it is frustrating to see people relax their guard now, because continuing to take measures that seem to be working, such as restricting indoor gatherings, may go a long way in helping end the epidemic. The herd immunity threshold “is not a’we are safe’ threshold, but a’we are safer’ threshold”. Even if the threshold is exceeded, isolated virus outbreaks will still occur.

The recent influenza epidemic has been greatly improved, which is the double superimposed effect of immunity and preventive behavior. Scarpino reminded that the flu may be as contagious as COVID-19. It is almost certain that the reason why the flu did not appear this year is that we usually have about 30% of the population immune because they have been infected in previous years, and Vaccination covers possibly another 30% of the population. So there is about 60% immunity in the group. Coupled with wearing a mask and maintaining social distancing, the flu will not break out. This rough calculation shows how behavior changes herd immunity and why more people need to be vaccinated to gain herd immunity when people stop implementing measures such as maintaining social distancing.

Stopping the spread of the virus is one of the ways to return to normal. But Stefan Flasche, a vaccine epidemiologist at the London School of Hygiene & Tropical Medicine, believes that another possibility is to prevent serious illness and death. Given the current understanding of COVID-19, it is quite impossible to achieve herd immunity through vaccines alone. Now is the time to put forward more realistic expectations. Flasche said that this vaccine is “absolutely shocking technology”, but it is unlikely to completely prevent the spread of the virus, so we need to consider how to coexist with the virus. This is not as scary as it sounds. Even without herd immunity, the ability to vaccinate vulnerable people seems to be reducing the number of hospitalizations and deaths due to COVID-19. Although the disease may not go away soon, its severity may diminish.

(source:internet, reference only)

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