Road to herd immunity: The hard truth

As the number of infections and deaths from Covid-19 increase, economies around the world have started to feel the heat. The situation in Bangladesh is no different, but it was terrifying to see hundreds of thousands of people flocking to the roads to go to their villages just to celebrate Eid very recently.  

The absence of public transportation as a part of the nationwide shutdown did not prevent people from following their deep desires to re-unite with their families. Despite the growing concerns of increased transmission and consequently higher risk of death from the virus, the authorities have declared that private vehicles can be used for transporting people home to rejoice in the holiday. Interestingly enough, most of the people wanting to rush home did not own a vehicle but became passengers of those who were willing to take them to their destination for an exorbitant amount, mostly defying the social distancing norms.

It is well understood that an economy like Bangladesh can't afford to sustain a long-term shutdown. The mounting pressure has already triggered relaxation in some economic sectors such as RMG in late April, shopping districts, etc.. However, during the time of rapidly increasing infections and fatality, the decision of relaxing the movements for non-economic activities or wholesale re-opening of businesses may not be well construed. Without denying the fact that the Eid celebration is deeply rooted into the socio-religious culture in this region, postponing the festivity or taking a more cautious approach in re-opening the economy wouldn't have inflicted more harm. Unfortunately, it is beyond doubt that the majority of the nation's population are neither fully aware of the consequences nor ready to follow any advisory. Therefore, a strict enforcement would have been warranted to keep everyone safe. For example, Saudi Arabia had imposed a curfew of five days during the Eid holidays to avoid transmission of the virus from mass gatherings.

The perceived relaxed attitudes towards the shutdown may pose a question of whether or not we have decided to pursue the route of so-called herd immunity. Before discussing this strategy in the present context, let's review what this really means. First, we examine how immunity works for us. Someone can become resistant after exposure to an agent that causes an illness such as Covid-19. The process of developing immunity includes producing antibodies specific to the virus for potential defence. A person is considered immune after a considerable amount of exposure to produce the antibodies irrespective of developing symptoms of the disease or without any evident symptoms.

Herd immunity can broadly be characterized as a condition in which large part of a population become immune to an infectiousagent, subsequently safeguarding others who lack immunity. For example, if 90 percent of a population is immune to a virus, nine out of every 10 people who encounter someone with the disease will not get affected and, as a result, will not have the ability to transmit the disease any further. According to experts, typically 50 percent to 90 percent of a population must be immune to achieve herd immunity based on how contagious the infection is. However, if the number of immune people drops lower than the threshold for a specific type of agent, the remaining people will have a higher chance of contracting the disease. Thus, the concept of herd immunity is based on establishing an extremely large group with immunity against an infectious agent by means of vaccinating or allowing people to get infected and recover. The immunity is then carried though a memory effect that produces antibodies when encountering that agent again. 

Some nations have been considering strategies intended to develop natural immunity to the coronavirus without a vaccine. For instance, Sweden, unlike its neighbours, decided to go for a broad immunity without implementing a shutdown. They let restaurants, bars, salons, gyms and schools remain open while promoting social distancing. Many have suggested, poorer economies like India or Bangladesh could also adopt this as a strategy to encounter the present outbreak. But they might have missed the math revealing a heavy toll with too many lives at risk. In fact, Sweden has already observed far more deaths from Covid-19 than its neighbours. Initially, their death rate was similar to the other European nations that shutdown their economies. But now, Sweden's mortality rate is the highest in Europe with a daily death rate of 8.71 compared to 4.59 of the USA per million people.

Experts predict at least 60 percent of the population will need to be immune in order to achieve herd immunity from Covid-19. To understand the statistic behind it, let's consider that one individual on average currently infects 2.5 others as observed by the experts. But, the containment of the pandemic requires limiting the effective transmission rate to 1, which indicates that enough people must develop immunity so that less than 1/2.5, i.e., 40 percent could remain susceptible to the infection. That means, herd immunity comes into effect when rest of the population (60 percent) become immune. In the absence of a vaccine, the outbreak would continue until we hit that magic number. The disastrous downside if a country like Bangladesh sees over a hundred million people infected, is to wait for at least 96 million of them to recover and become immune to the virus. It is almost impossible to predict what that will mean in terms of human costs, but we are looking at 100,000's of fatalities, considering the nation's present rate of 1.4 percent fatality from the virus.

Even if we are willing to consider the natural route for herd immunity, accepting its extremely high human toll, we must be mindful of our capabilities in handling the huge implications of our country's poor functioning medical system with 10-15 percent of all infections requiring hospitalisations. The hard truth is herd immunity through the infection route isn't the answer to our current despairs. However, the required threshold could be lowered by reducing the number of people possibly infected by one individual. For example, if we could lower the effective transmission rate to 1.5 per person, herd immunity would come into effect when about a third of the population has had the infection and has become immune.

Although herd immunity would be the ultimate goal in the quest for recovery, it would be an unrealistic expectation to stop the epidemic before we have access to a vaccine.On the other hand, the transmission of virus can be limited by avoiding contact with other infected individuals. Driving down the number of contacts will in effect reduce the possibility for an infected individual to transmit a virus and hence herd immunity can start earlier.

Dr Sabbir Ahmad is a researcher, mentor and a leader in project delivery and engineering.