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 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.
Graphic: How a vaccine slows the spread of a virus https://graphics.reuters.com/HEALTH-CORONAVIRUS/HERD%20IMMUNITY%20(EXPLAINER)/ygdvzmqqgpw/index.html
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.
"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.
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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.
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."
(Writing by Jane Wardell; Editing by Stephen Coates)
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