Many viral infections, although extremely common and occasionally deadly prior to the advent of vaccines against those ailments, have become rare through herd immunity imparted by mass immunization.
Infections, when no vaccine is available, can continue to infect children and the other vulnerable groups even when many adults develop immunity by prior exposure. Many other viruses mutate, so that immunity developed from the previous infection becomes short-lived such as that against the flu virus.
In case of SARS-Cov-2 virus, there are a number of situations that could be encountered. If indiscriminate social mixing was pursued, the virus would have infected a large proportion of the population, overwhelming the medical care systems, resulting in high and unacceptable human fatality. Another option would be to adhere to strict social distancing to reduce the rate of infection until a vaccine that is effective and safe is made available.
There is a middle path of controlling the rate of infection to fall and rise over time, relaxing social distancing measures when infections fall, and re-implementing measures when the number increases. It appears that eventual herd immunity through vaccination is the way forward in the battle against SARS-Cov-2 virus.
Epidemiologically, a herd immunity is the state where an entire population becomes immune to a disease, though only a substantial portion of that populace has been vaccinated or has gained natural immunity from infection. Herd immunity hampers the ability of the people to transmit a disease within a group, and protects a population from a disease after an organism has infected enough people in that community.
This is more commonly seen after vaccination. Vaccine imparts immunity that stops the pathogen from transmitting the disease from one person to another, irrespective of their vaccination status. They include members of the community who cannot be or have not been vaccinated.
Vaccines provide direct protection of vaccinated individuals, and at the same time indirect additional protection of non-vaccinated individuals. Vaccine-induced immunity reduces the prevalence of susceptible individuals in the population and reduces the probability that an infectious case will come into contact with a susceptible person.
Another phenomenon of herd immunity in addition to the reduction of the prevalence of infectious cases, immunity imparted could reduce the severity in and infectivity of individuals.
Herd immunity only works for diseases that are contagious and spreads directly between people like flu, measles, mumps, cholera. Herd immunity does not work in the case of tetanus. The tetanus bacteria, clostridium tetani, lives in the soil, so anyone who is not vaccinated is susceptible.
When a person who is not immunized against tetanus has a dirty wound and that wound gets contaminated by soil containing tetanus organism, the person may easily develop tetanus, even when everyone else around was vaccinated and protected.
Herd immunity achieved through infection or immunization may vary in a number of significant ways and would determine the policy of combating an infection responsible for a severe epidemic and/or a pandemic.
The majority of people’s understanding of immunity is that once a person has suffered from an infectious disease, usually, though not necessarily, they can’t get the disease again. Is it possible then in Covid-19 pandemic, that if enough people were infected by the virus, there would be widespread immunity in the community that would protect everyone around?
The quantum cost of herd immunity without vaccination would depend on the infection’s contagiousness, virulence, and the amplitude and the duration of protective immunity imparted, among others.
Each variable is intimately dependent on the other and even a minor shift in the individual variable would drastically and dramatically alter the outcome pertinent to death and destruction. The principal variables for Covid-19 are the R0 (contagiousness) -- the basic reproduction number; the average number of people a contagious person will infect, when no one is immune and people make no effort to stop the spread. R0 of 3 means that an infected person is capable of infecting on an average another three people in the community. The higher the R0, the higher the proportion of the population will be required to be immune to stop the spread of infection.
Death rate (virulence) is the percentage of people who die from the infection. The duration of immunity (immunity imparted) is the length of time an individual remains immune after recovering from the disease.
The more people an infected individual can infect, the larger the number of people needed (to be infected/immunized) to impart herd immunity to be effective (threshold for the herd immunity). The R0 for measles, for example, is 12 to 18, meaning that a large proportion of the population, say about 95%, will need to be immunized to attain herd immunity, often the reason for measles outbreak consequent on the minutest fall in immunization drive against measles.
However, the rate of spread would determine how quickly the threshold is arrived at. A rapid and fast herd immunity threshold would also mean a huge proportion of the population becoming unwell simultaneously, overwhelming the health care system to cope, and unless the infection is relatively of low, virulence and the death rate is extremely low, fast achievement of herd immunity would create undesired devastation.
If the duration of the acquired immunity is short and the infection is still active, the individuals in the population would become susceptible to infection again. The R0 for Covid-19 is not fully established, but is estimated to require a threshold for 60-90%. An unusually huge number of people of a population would be required to be infected to provide herd immunity and with even a mortality rate of 0.5 to 1%, the death rate will simply not even be discussable.
A herd immunity achieved through infection by SARS-Cov-2 virus of 60% of Bangladesh’s population is an unimaginable proposition. In New York City, with a population of 8.3 million, there were roughly around 200,000 confirmed cases with a death rate of 10% (1 in every 400 people), and the medical service was overwhelmed by the rapid surge in infected patients. An infection of 60% of New York would have caused a staggering number of deaths of New Yorkers.
Sweden appears to have set a tempting example of allowing most businesses to stay open, relying on the strategy of achieving herd immunity, allowing a critical mass of infection to occur in a proportion of the populations that prevent transmission. Swedish public health experts, social scientists, and media is pretty vocal in advertising success of its policy of Covid-19 combat strategy, avoiding lockdown in order to achieve herd immunity.
They believe and advocate that Sweden, which had encouraged voluntary social distancing but did not pursue full lockdown, could guide the world. The authorities in Sweden claim the country is rapidly approaching herd immunity, a claim much contradicted by the rest of the world. In fact, Sweden’s policy has been inconsistent throughout, their fatality figure has been worse than their neighbours, there has been scathing criticism of their failure to protect vulnerable populations, particularly the elderly, and their estimation of immunity is at worst misleading or too early to conclude at its best scenario.
So far, we know, herd immunity can only be reached either by widespread vaccination or by individuals falling ill and recovering, thereby developing natural immunity against the virus. Unfortunately, the disease causing the pandemic is too deadly, too contagious, and it has not been with us for long enough to confidently predict how effectively and for how long the immunity would last once people have recovered from the disease and naturally acquired immunity is not a feasible option at all.
Dr Raqibul Mohammad Anwar is Specialist Surgeon and Global Health Policy and Planning Expert, and Retired Colonel, Royal Army Medical Corps, UK Armed Forces.
