The CoviD-19 saga: Herd Immunity

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When a large percentage of a population has become immune to an infection, whether through previous infections or vaccination, they may provide a measure of protection for individuals who are not immune. This is known as herd Immunity, also known as herd effect, community immunity, population immunity or Social immunity.

In a population plagued by a contagion, everyone contributes to the spread of the disease, except for those who are immune. If a large enough proportion of individuals possess immunity, it may disrupt chains of infection, which slows or probably stop the spread of disease. The greater the proportion of immune individuals in a community, the smaller the probability that non-immune individuals will come into contact with an infectious individual, helping to shield non-immune individuals from infection.

When a critical portion of a community is immunized against a contagious disease, most members of the community are protected against that disease because there is little opportunity for an outbreak. Even those who are not eligible for certain vaccines – Infants, pregnant women, or immunocompromised individuals. They get some protection because the spread of disease is contained.

In the illustration, the top box depicts a community in which no one is immunized and an outbreak occurs. In the middle box, some of the population is immunized but not enough to confer herd immunity. In the bottom box, a critical portion of the population is immunized, protecting most community members.

The principle of herd immunity applies to control of a variety of contagious diseases, including influenza, measles, mumps, rotavirus, and pneumococcal disease.

Once a certain threshold has been reached, herd immunity gradually eliminates a disease from a population. This elimination, if achieved worldwide, may result in the permanent reduction in the number of infections to zero, called eradication. An example would be smallpox, eradicated in 1977 through mass vaccination. Herd immunity does not apply to all diseases, just those that are contagious, meaning that they can be transmitted from one individual to another. Tetanus, for example, is an infectious disease but not contagious, so herd immunity does not apply.

Achieving Herd Immunity: Is it Possible to reach it before a Vaccine?

A paper from the Lancet says a study of SARS-CoV-1 survivors showed about 90 percent had virus-neutralizing antibodies and about half had strong responses from another immune system warrior, the t-cell. So this observation is hopeful. Though, when a grim scenario happens that a vaccine is not possible and no antiviral treatments emerge to treat the most deadly symptoms of CoviD-19, countries would have no choice but to follow Sweden’s model of trying to slowly expose the whole population in the hope of producing herd immunity.

But that won’t come without a cost. According to data from John Hopkins University, Sweden currently has a death rate of 297.16 for every 1 million citizens, after 3,040 died from the novel coronavirus. Sweden’s lockdown-free approach has seen their death toll far exceed that of their Scandinavian neighbours like Denmark (514), Norway (217) and Finland (255) who have imposed strict measures to try and contain the spread of CoviD-19.

The percentage of people who need to become immune in order to confer herd immunity varies from disease to disease. Gideon Meyerowitz-Katz, an epidemiologist, said that a reasonable estimate for CoviD-19’s herd immunity threshold is 60-70 percent of the population. The CDC’s data said that the number could be more like 85 percent. Many people are attempting to use this herd immunity solution as a way to go back outside with the notion of simply letting SARS-CoV-2 spread unmitigated until more than 70 percent of the globe are infected, and after recovery produce herd immunity. That would also mean sacrificing hundred millions of lives because not all may be able to recover, and we still lack evidence over the assumption that people who get CoviD-19 once cannot get it again.

Herd Immunity through Vaccination

As of April 2020, it is unknown if past infection provides effective and long-term immunity in people who recover from CoviD-19. Based on what we know of coronaviruses, acquired immunity is highly likely, and some infected have been reported to develop protected antibodies. Cases in which recovery from CoviD-19 was followed by positive tests in later dates have been reported but these cases are believed to be lingering infection rather than reinfection or false positives because of remaining RNA fragments. Based on what we know, coronaviruses are capable of reinfection in roughly a year.

On March 11, 2020 the WHO said that the pandemic can be controlled, but the peak and ultimate duration is uncertain and may differ by location. According to the Imperial College study led by Neil Ferguson, physical distancing and other measures will be required until a vaccine becomes available, which is potentially 18 months or more. William Schaffner of Vanderbilt University said that it’s unlikely that this coronavirus – because it’s so readily transmissible – will disappear completely and has a chance to turn into a seasonal disease, making a comeback every year. With the virulence of the comeback dependent on herd immunity (including vaccinations) and the extent of its mutation.

Think of common respiratory viruses that cause the flu or the common cold, we may receive vaccinations every year and still come down with one of these viruses by the next cold season. We aren’t perfectly protected against them, as there are literally hundreds of viruses that cause colds, but having been exposed to at least one strain may make subsequent infections less serious. Immunity can also wear off over time, which is why booster shots are common to prevent infections such as tetanus. Other shots generally last for life, like the MMR vaccine for mumps, measles and rubella. If a COVID-19 infection offers partial protection and we are able to develop a vaccine that is also partly protective, as is the case with the flu, we could still achieve a form of herd immunity. But despite the lingering uncertainties, Experts emphasized that a vaccine will be crucial to achieving widespread protection from SARS-CoV-2 in the future.

Opposition to vaccination has posed a challenge to herd immunity, allowing preventable diseases to persist in or return to communities that have inadequate vaccination rates. And even in the hopeful event of apparent elimination through herd immunity and vaccination, SARS-CoV-2 surveillance should be maintained since a resurgence in contagion is still possible as late as 2024.

REFERENCE:

Introduction to Herd Immunity

Herd Immunity and the Coronavirus Disease 2019

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