Explainer: Reaching Herd Immunity In A Viral Pandemic

There are two pathways to herd immunity: natural infection or vaccination

Sydney: The novel coronavirus pandemic has brought “herd immunity” to the public consciousness, kindling hope the phenomenon can help slow or even end the outbreak. Herd immunity refers to a large portion of a community developing a degree of immunity to a virus, thereby reducing person-to-person spread. As a result, the whole community gains protection, not just those who are immune.

NATURAL INFECTION VS VACCINATION

There are two pathways to herd immunity: natural infection or vaccination.

Natural infection refers to when a large number of people have had a disease and recovered. However, the extent of protection via natural infection is unknown with the new coronavirus. Moreover, more people would die while waiting for herd immunity than if a vaccine was produced.

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The risk is not acceptable, said Catherine Bennett, epidemiology chair in the Faculty of Health at Melbourne’s Deakin University. We can’t afford to have people infected to reach herd immunity when we know so little about the longer-term effects.

Vaccination can provide widespread immunity faster and more reliably.

There is no vaccine for COVID-19 – the disease caused by the novel coronavirus – though trials at different stages are underway around the world. It usually takes several years for a vaccine to be identified, tested, produced and distributed for public use. Vaccine makers hope to dramatically compress that timeline for COVID-19 through faster trials and by manufacturing at scale even before products have proved successful.

Experts believe if no other measures are taken, herd immunity could kick in when 50% to 70% of a population gains immunity through vaccination. The precise level depends on the vaccine’s efficacy rate, which experts say will be 70% at best.

BALANCING VACCINE DISTRIBUTION

How a vaccine is distributed has implications for effectiveness. If shared unevenly – for example, if the wealthy have greater access than those in poorer locations – that would create safe clusters but leave large areas of susceptible people.

In the early stages of distribution, higher priority may be given to healthcare workers and others on front lines, or those considered most vulnerable – a process known as targeted vaccination. That risks missing people who might be considered “super spreaders”, such as public transport workers.

We need to be sure that we spread the vaccine equitably through the population, said Joel Miller, a senior lecturer in applied mathematics at La Trobe University in Melbourne, who uses mathematical models to help governments and non-profit organisations formulate policies to control infectious diseases.

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MOVEMENT RESTRICTIONS

The movement of people also has implications for the spread of a virus.

At lower vaccination levels, the number of people who eventually become infected is similar in a group of people who mix and travel widely, and a group of people who are relatively static. However, the spread is much slower in a static population, hence governments worldwide imposing lockdown measures.

Even when a high percentage of the population is vaccinated, infection numbers can be reduced further if people refrain from travel.

WHILE WE WAIT

The new coronavirus is spread primarily via droplets expelled when a person coughs, sneezes or even talks.

Until a vaccine is developed, wearing masks, physical distancing and hand hygiene can help reduce transmission and contribute to creating herd immunity.

Epidemiologists largely agree that a combined approach is critical given early vaccines brought to market will likely not have 100% efficacy.

It’s about adding layers, said Deakin University’s Bennett. It gives us extra protection from community spread. The situation is very much better in places where a combination of measures is being used.

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(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)

NDTV – Dettol Banega Swasth India campaign is an extension of the five-year-old Banega Swachh India initiative helmed by Campaign Ambassador Amitabh Bachchan. It aims to spread awareness about critical health issues facing the country. In wake of the current COVID-19 pandemic, the need for WASH (Water, Sanitation and Hygiene) is reaffirmed as handwashing is one of the ways to prevent Coronavirus infection and other diseases. The campaign highlights the importance of nutrition and healthcare for women and children to prevent maternal and child mortality, fight malnutrition, stunting, wasting, anaemia and disease prevention through vaccines. Importance of programmes like Public Distribution System (PDS), Mid-day Meal Scheme, POSHAN Abhiyan and the role of Aganwadis and ASHA workers are also covered. Only a Swachh or clean India where toilets are used and open defecation free (ODF) status achieved as part of the Swachh Bharat Abhiyan launched by Prime Minister Narendra Modi in 2014, can eradicate diseases like diahorrea and become a Swasth or healthy India. The campaign will continue to cover issues like air pollution, waste management, plastic ban, manual scavenging and sanitation workers and menstrual hygiene.