By Charles Nicolson

A collective defence against the Covid-19 virus.

Now that effective vaccines are available to counter the current variants of the Covid-19 virus, the war against the dangerous disease caused by this virus is going to be won, although it will probably need additional time if viral mutations continue to produce new variants.

The actual time since the outbreak became apparent is surprisingly short. It has taken (at the time of writing) only 14 months to go into history as one of the worst – perhaps the worst pandemic ever – to occur.

When this highly infectious coronavirus first started to spread, virtually nobody was immune. Consequently, the virus spread rapidly across communities. In May 2020, ten months ago (at the time of writing again), the city of Manaus in Brazil experienced a sudden, large outbreak of Covid-19.

However, after three months the situation had changed. By August 2020, despite having reduced social-distancing which had been introduced at the beginning of June 2020, the number of deaths per day in this city of some 2-million people had reduced from over a hundred to less than ten.

Researchers suggested that one of the main reasons was because a large proportion of the community’s population had probably experienced early individual exposure to the virus which enabled them to develop immunity.

“Once a sufficient proportion of the population is no longer susceptible to the virus, any new outbreak diminishes and then peters out.”

Immunologist Ester Sabino at the University of São Paulo, Brazil, reported test results for antibodies to SARS-COV-2 (the virus that causes Covid-19) on 6 000 samples from blood banks in Manaus showing that the number of infections resulting from the first wave reached nearly 70%. Sabino proposed that this high infection rate meant that the number of people who remained vulnerable to the virus reduced rapidly to a level too small to sustain any new outbreaks; a situation known as ‘herd immunity’, a term used in previous viral outbreaks for example of measles, mumps, or going back to the massive flu pandemic in 1918.

Clearly, achieving herd immunity against any hostile virus is a desirable objective and many methods of achieving this have been proposed. Initially, a basic theme to simply leave the coronavirus to run its course was suggested allowing people to return to their normal daily living while precautions were implemented to protect those at higher risk levels.

However, specialist epidemiologists were vigorously opposed to this approach insisting that it would undoubtedly result in needless loss of life. Kristian Andersen, an immunologist at the Scripps Research Institute in La Jolla, California, has stated that. “Surrendering to the virus is not a defensible plan, we have never managed to do this successfully before for maladies such as mumps and polio and it will lead to unacceptable and unnecessary deaths and suffering without speeding up the return of society to normal”.

Once a sufficient proportion of the population is no longer susceptible to the virus, any new outbreak diminishes and then peters out. “You don’t need everyone in the population to be immune – you just need enough people to be immune,” says Caroline Buckee, an epidemiologist at Harvard T.H. Chan School of Public Health in Boston, Massachusetts.

Many medical professionals do not like the term ‘herd immunity’ and prefer to call it ‘herd protection’. Buckee goes on to say that “The phenomenon doesn’t actually confer immunity to the virus itself – it only reduces the risk that vulnerable people will come into contact with the pathogen”.

Previously achieved herd immunity for other infectious diseases

Measles, mumps, polio, and chickenpox are prime examples of infectious diseases that were very common but are now rare mainly because vaccines have been developed which have been effective in establishing herd immunities.

However, there have been sporadic outbreaks of vaccine-preventable diseases in communities with lower vaccine coverage because, at the times of infections, they did not have sufficient protection. A prime example of this is the measles outbreak at Disneyland in 2019.

Viruses tend to mutate so antibodies from previous infections provide protection for only relatively short periods of time, for example protection against flu is usually less than a year. If SARS-COV-2, the virus that causes Covid-19, is like other coronaviruses that currently infect humans, it can be expected that people who have been infected will be immune for months or possibly years, but probably not for their entire lives.

There are two main methods of achieving herd immunity: either a sufficient proportion of the population becomes infected or is deliberately dosed with a protective vaccine. At worst, the virus can infect so many people in just a few weeks that hospitals could be overwhelmed with consequential high death rates as happened at Manaus in Brazil.

At best, current levels of infection are maintained or even steadily reduced until a vaccine becomes available. However, continued physical distancing for an extended period of a year or longer on the part of the entire population is often needed.

Restrictive social distancing measures can only be reduced after the number of infections has declined, but they may need to be re-implemented if numbers increase again. Even so, SARS-COV-2 can still infect children before they are vaccinated or adults after their immunity has waned. However, in the longer term, the explosive spread that has happened recently is unlikely because much more of the population will be immune by then.

Similarly, the earlier description as to why letting herd infections ‘just happen’ is not a good idea, deliberately getting infected with SARS-COV-2 to ‘get it over with’ is not a course which should even be contemplated.

Previously, for some other diseases, such as chickenpox before the varicella vaccine was developed, people did sometimes expose themselves intentionally as a way of achieving immunity. Generally, for less severe diseases, this method might be acceptable but when SARS-CoV-2 is involved any Covid-19 content includes much higher risks of disease, or death.

Anticipated death rates for Covid-19 have not yet been fully determined but current data suggest they will be around 10 times higher than for the flu and higher still where vulnerable groups like the elderly and people with weakened immune systems are involved.

As already mentioned, the current physical distancing measures will not always need to be as strict as the present ‘shelter-in-place’ directives are. Thanks to extraordinarily intense actions already taken on a global scale, encouraging news in the overall grim Covid scenario is that newly developed and tested vaccines are now being widely distributed with their effectiveness fully maintained by another modern technology – reliable refrigeration.