Herd Immunity and Covid-19 Metamorphosis
With various levels of vaccine efficacy ranging from 60-94 percent, asymmetric vaccination results and the development of various virus strains, the government must recalculate a rational vaccination target.
Covid-19 vaccination programs are currently underway massively around the world. About 550 million doses of vaccine have been injected into about 4 percent of the world\'s population. Roughly, more than 2.5 million doses of vaccine are given each day.
Several countries are even speeding it up through crash programs. They want herd immunity to be achieved as soon as possible. In Gibraltar, as much as 92 percent of the population has been vaccinated, in Israel 60 percent, Britain 44 percent and Northern Ireland 38 percent.
Covid-19 vaccination does have two substantial objectives, namely individual protection and community protection or herd immunity. After being vaccinated, people will form antibodies that protect themselves from Covid-19. However, this protection is not 100 percent; depending on the efficacy of vaccines and the body\'s ability to build immunity. This means that people who have been vaccinated still have a chance of becoming infected, albeit a small one.
To maximize protection and stop disease transmission, the surrounding people must also have immunity. A number of experts agree that herd immunity can only be achieved when 70 percent of the population has acquired immunity, either because they have been infected or because they have received a vaccine. If this level is not reached, the pandemic will continue.
In the history of disease outbreaks, herd immunity has often become the principle of prevention. This principle refers to a situation in which the spread of a disease can stop if a large number of the population in an area has been infected with the disease or has been vaccinated. From the disease and vaccine, the body builds antibodies that play a role in fighting infection.
Also read: Vaccine-induced and Natural Immunity
The phenomenon of herd immunity was first reported in 1923. At that time, experts observed a significant decrease in measles sufferers in Baltimore, which occurred after most children were infected with measles. This principle then became the mainstay for the prevention of disease outbreaks. In each outbreak, the concerned party strives to achieve herd immunity through vaccination.
In reality, these efforts have paid off. Various infectious diseases, such as polio and measles, have now been eradicated as a result of attaining herd immunity to these diseases. Learning from that experience, now people pin their hopes on this phenomenon for Covid-19. Even though many hope that herd immunity can control the current pandemic, a number of experts remind us of the need to exercise caution in relying on this principle.
First, the herd immunity threshold turns out not to be constant. The 70 percent calculation is not the standard figure and universal threshold for herd immunity. This threshold can change from one country to another and from one vaccine to another.
Many factors influence it. The current standard for calculating herd immunity uses the effective reproduction number (Rt) parameter, namely the number of people infected by a person with Covid-19. This value is obtained from the results of tests and surveillance. Initial estimates suggest that the Rt of Covid-19 ranges from 2.5 to 3.0. This means that one infected person has the potential to infect 2-3 other people.
The herd immunity threshold turns out not to be constant.
To control the pandemic, this Rt value must be kept below 1. One of the main strategies is to provide a vaccine with a 90 percent efficacy to about 74 percent of the population. If the efficacy of the vaccine is less than 90 percent, the population covered must be larger. Based on the simulation analysis, if the effectiveness of the vaccine is in the range of 60-70 percent, the vaccination coverage should be in the range of 80-90 percent of the population.
This coverage threshold needs to be calculated accurately. For Indonesia, which uses the Sinovac vaccine with an efficacy level of 60-70 percent, the vaccination coverage target should not be 70 percent, or 180 million people. It must be above that. At least 80-90 percent. The scenario is different if other vaccines are used. The higher the vaccine efficacy, the lower the target vaccination coverage will be.
Also read: The Pursuit of Herd Immunity
Second, the current Covid-19 vaccines have an efficacy of 60-94 percent. However, it is not yet known how long this efficacy can last. Is it three months, one year, or five years? It is still being researched. The duration of vaccine efficacy is a very important factor in achieving herd immunity and pandemic control. If the vaccine efficacy is less than one year, it will be difficult to achieve herd immunity in a short time.
Herd immunity requires the presence of a population that is protected together in a certain period, sufficient to limit disease transmission. If the efficacy is less than one year, vaccination should be repeated at least annually. One of the reasons measles was able to be eradicated is because of the availability of a measles vaccine that has a long duration of efficacy.
The measles vaccine is given twice. The first dose provides 93 percent protection and the second dose 96 percent. The interval between the first and second doses is quite long, namely 3-5 years. After receiving the two doses, people will be immune to measles for decades or even for life. With this level and duration of efficacy, as well as the large vaccine coverage, it is natural that measles has now been eradicated.
Third, herd immunity can be achieved if a large number of the population becomes immune simultaneously within a certain period. Herd immunity is not achieved if immunity happens in a scattered or asymmetrical way. For example, one area gets vaccinated and another area does not, or one area is vaccinated today and another area 1-2 years later.
Also read: Vaccine and Vaccination against Covid-19
Scattered or asymmetrical immunity has the potential to trigger the reappearance of infection. Say, in the last six months 70 percent of the population of West Java has been given a vaccine with an efficacy that lasts a year, while 70 percent of the population of East Java just finishes vaccination in the next 1-2 years. This would not result in effective herd immunity because as soon as the East Java population achieves herd immunity, the West Java population begins to lose the effectiveness of the vaccine. This consideration is the basis for a number of vaccination programs to be carried out simultaneously, such as national immunization week.
It is also on these considerations that the UK has a different policy regarding vaccination programs. Rather than giving two doses of vaccine to a limited number of the population, the UK has chosen to give one dose of vaccine to as many people as possible. The administration of the second dose is postponed to three months, rather than 3-4 weeks, after the first vaccine. In principle, they choose the option that as many people as possible receive one dose of vaccine rather than a minority of the population who receive two doses of vaccine. They want to pursue herd immunity as quickly and as widely as possible.
The ability to mutate is a natural process for many types of viruses.
Fourth, the development of the Covid-19 virus mutation. Similar to other viruses, Covid-19 has the ability to mutate and form new strains. The ability to mutate is a natural process for many types of viruses. The number and type of mutations is directly proportional to the spread of the disease; the higher the spread of the disease, the higher the possibility of the mutations and virus strains will be.
The types and effects of mutations vary. One of them, the virus modifies its spike protein which allows it to attach more tightly and longer to the cells of the human body. Finally, the virus transmission also increases. In just a few months, the first mutated strain of Covid-19 which was discovered in the UK, namely B.1.1.7, has dominated 60 percent of new Covid-19 cases in the UK. Another strain was found in Brazil, namely B.1.351. This type is able to infect people who have been infected with Covid-19 and already have antibodies.
In essence, viral mutations can affect the spread and severity of the disease, as well as affect the herd immunity threshold. This means that the greater the number of virus mutations, the higher the vaccination target needed to achieve herd immunity.
Based on the above arguments, several experts expressed doubt that effective herd immunity can be achieved. Moreover, according to them, this pandemic has been going on for a long time. It is difficult to control a pandemic if the disease transmission has been going on for a long time. On this basis too, they hypothesized that the world will never achieve global herd immunity and will only experience asymmetrical herd immunity.
Also read: Intensifying Covid-19 Genomic Surveillance
This means that several countries will achieve local herd immunity and several others will not achieve any herd immunity and remain vulnerable to the spread of Covid-19.
Countries with local herd immunity are indeed temporarily protected from Covid-19. However, this immunity is partial and incomplete. The reason is that countries with local immunity will continue to interact with countries that have not been protected. This interaction among the population has the potential to cause recurrence, reinfection or advanced virus mutations.
As a result, Covid-19 will never completely disappear from the Earth. This disease will disappear and appear depending on global interactions, changes of seasons or weather, and the management that is carried out. When winter arrives in Western countries, intense interaction among the residents and relaxation of pandemic protocols is inevitable, and Covid-19 will appear again. This disease metamorphoses into an endemic disease. It appears at a certain time and place, disappears after being handled and reappears at another time in another place. Roughly resembling the current pattern of seasonal flu or influenza.
The opinion of the experts above is still just a hypothesis. It may be right, it may be wrong. However, this hypothesis is not without rational reasons. That means, it is not impossible for this hypothesis to be true. So it is highly necessary to take action to ensure that this hypothesis does not become true.
Also read: Responding to the New Covid-19 Variant
All countries should strive to increase vaccination coverage. Vaccinations are now quite widespread, but are not yet effective in controlling the pandemic quickly. At the world level, 2.5 million people, or 0.03 percent of the population, are vaccinated every day. In Indonesia, two months after the vaccination program began, more than 7 million doses of vaccine have been given. Every day, more than 110,000 doses of the vaccine are injected. It is massive, but not yet adequate. At the current rate, it will take 5-8 years to achieve herd immunity. This rate needs to be accelerated at least twofold.
With various levels of vaccine efficacy ranging from 60-94 percent, asymmetric vaccination results and the development of various virus strains, the government must recalculate a rational vaccination target. Do not make 70 percent the absolute number to be achieved. The dynamic changes of the above parameters necessitate an increase in the number of vaccination targets.
A number of countries in the world are targeting immunization coverage above 70 percent. Gibraltar and Israel even target all of their population to be vaccinated, except for those with contraindications. For Indonesia, this is not impossible, especially now that 11 types of Covid-19 vaccines have received approval for use and dozens of others have the potential to be approved in the coming months.
It is simply up to us to be smart in negotiating with vaccine manufacturers and make efficient use of the vaccines we have.
Iqbal Mochtar, Physician and Observer of Health Issues.
This article was translated by Hyginus Hardoyo.