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Herd immunity: the myths & realities

For last few weeks, the concept of herd immunity is making headlines in the international media with reference to COVID-19. Herd immunity is an epidemiological term usually reserved to describe how the population as a whole is protected from a disease depending on the levels of people vaccinated.

What it is?

Herd immunity (or community immunity) occurs when a high percentage of the community is immune to a disease (through vaccination and/or prior illness). So it is unlikely that disease will spread from person to person. The people not vaccinated (such as newborn and the immunocompromised) are offered some protection because the disease has little opportunity to spread within the community.

Concept of herd immunity

The logic of herd immunity is deceptively simple. The antibodies that our bodies need to fight off the infection are developed through two ways:

(i) By getting the infection and allowing our body to develop antibodies naturally.

(ii) Through vaccination whenever the vaccine is developed.

When a person is infected with a virus, the body produces antibodies that fight off the infection and the person recovers. After the recovery, he becomes immune to the virus which means he cannot get it again and cannot infect any other person. If 70 percent of the population of a country therefore is infected with the virus, the remaining 30 percent will have very low probability of getting infected. This is because the 70 percent who are infected and recover are not passing on the infection.

The Sweden scenario

Sweden has broken away from international norms and has not instituted any formal lockdowns. It has, however, strongly advised its citizens to practice social distancing and asked older citizens to stay at home while clamping restrictions on access to nursing homes.

Vaccines prevent many dangerous and deadly diseases. In the United States, smallpox and polio have both been stamped out because of vaccination. However, there are certain groups of people who cannot get vaccinated and are vulnerable to disease: babies, pregnant women, and immunocompromised people, such as those receiving chemotherapy or organ transplants. Measles was declared eliminated in 2000. Yet in 2014, there were 668 cases reported. The disease was spread when infected people traveled to the United States. These infected people then exposed unprotected people to the disease. There are a number of reasons why people are unprotected: some protection from vaccines ‘wanes’ or ‘fades’ after a period of time. Some people don’t receive all of the shots that they should to be completely protected. For example you need two measles, mumps and rubella (MMR) injections to be adequately protected. Some people may only receive one and mistakenly believe they are protected. Some people may object because of religious reasons, and others are fearful of potential side effects or are skeptical about the benefits of vaccines.

Drawbacks of herd immunity

One of the drawbacks of herd immunity is that people who have the same beliefs about vaccinations frequently live in the same neighborhood, go to the same school, or attend the same religious services, so there could be potentially large groups of unvaccinated people close together. Once the percentage of vaccinated individuals in a population drops below the herd immunity threshold, an exposure to a contagious disease could spread very quickly throughout the community. However, the World Health Organization has condemned the ‘dangerous’ concept of ‘Herd Immunity’ for COVID-19.


WHO’s instance

Asked about the concept being applied to the COVID-19 pandemic, the World Health Organisation said “no-one is safe until everyone is safe” and it is “dangerous” to think that countries can “magically reach herd immunity”. Dr. Mike Ryan, executive director of the WHO’s health emergencies program, warned that “This is a serious disease, this is public enemy number one, we have been saying it over and over and over and over again.” He said “no one is safe until everyone is safe”. He further said “So I do think this idea that maybe countries who had lax measures and haven’t done anything will all of a sudden magically reach some herd immunity, and so what if we lose a few old people along the way?”

The technical head of WHO’s COVID-19 response Dr. Maria Van Kerkhove, has said that the preliminary data from studies has shown that very low levels of the population have actually been infected with the illness. There seems to be a consistent pattern so far, that a low proportion of people have these antibodies. Nobody knows that what that level needs to be for COVID-19. But it is clearly needs to be higher than what we’re seeing in zero-prevalence studies; which indicates to us is that there is a large portion of the population that remains susceptible.”

The Pakistan scenario

In Pakistan, no official admits that herd immunity has been discussed as a policy option.

“Absolutely not,” says Dr Zafar Mirza, Special Assistant to the Prime Minister on Health and has clearly mentioned that this is not government policy. He forcefully said that we are neither promoting herd immunity nor discussing it at all.

There is a perception that the relatively low numbers of deaths have helped them proceed with relaxation of the lockdown. These low numbers may denote that the lockdown in the early stage was effective and suppressed the infection rate or that somehow Pakistanis have greater resistance to the infection. Although there is no conclusive scientific proof yet. Furthermore it has been observed that the trajectory of infections in California and Texas in the United States has remained much lower than in New York. These two states have warmer weather than New York. New Delhi and Karachi have similar infection spreads, but New Delhi has a lower death rate than Karachi. Interestingly it has been observed that wherever the temperatures are higher, the impact of the virus is lower.

The Pakistan’s current population is around 220 million; 70% of which is around 155 million. So around 155 million people would be required to be infected for herd immunity. In simple words 15 crore Pakistanis will require to be infected with COVID-19 before herd immunity kicks in. On the other hand the current mortality rate of Pakistan is 2.2%. This is the figure which explains that people dying from among all those infected. But the actual mortality rate will be lower because the confirmed cases of infections are far less than what the actual number of infected people. The actual figure of infections in Pakistan could be anywhere from double the confirmed figure to ten times upwards. This provides us a wide range to apply on the concept of herd immunity. Since the current mortality rate of the confirmed infections is 2.2 per cent, doubling the number of infected cases would bring down the projected mortality rate to 1.1 per cent. If we increase the actual cases ten times, the mortality rate would reduce by ten times and end up at a figure of 0.22 per cent. Therefore, the range to apply to the herd immunity level would be 0.22-1.00 per cent.

Last word

A good place to start is by dispelling the myths about herd immunity. We must understand that how herd immunity works and when it does not. We have to keep this into mind that the consequences to the larger community of the supposedly “individual” choice to forgo or delay immunizations.

It is too early to tell whether the approach has worked. Even if we had perfect knowledge of the Swedish case, there are huge risks with copying the strategy in a country like Pakistan. The Pakistanis are far less healthy than Swedes. They have significantly higher rates of diabetes and hypertension, two of the most-risky underlying conditions.

The key will be for countries like us is to be vigilant. They must roll out a testing and contact-tracing infrastructure that will allow them to identify outbreaks early and isolate and quarantine as necessary. In Pakistan, this is an unrealistic goal as we have lack of enough political willpower, poor economic conditions, illiteracy and no apparent effective coordination among provinces. These things, and not Sweden’s experience, should guide our next steps.

The simple fact is that herd immunity, without a vaccine, is not an easy way forward. Ultimately, what we do next will be more a political than a scientific decision. Herd immunity may still sound idyllic, but it requires us to sacrifice the vulnerable on the altar of the economy in truly vast numbers.

So the best thing is stay inside as far as possible.

The author, Nazir Ahmed Shaikh, is a freelance columnist. He is an educationist by profession and writes articles on diversified topics. He could be reached at [email protected].

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