Explainer: What is herd immunity and will it affect the pandemic?

The outbreak of disease caused by the new coronavirus has raised questions about a phenomenon known as "herd immunity" and whether it might play a role in how the pandemic progresses.

Here are some insights from infectious disease specialists:

WHAT IS HERD IMMUNITY?
* Herd immunity refers to a situation where enough people in a population have immunity to an infection to be able to effectively stop that disease from spreading.

* For herd immunity, it doesn't matter whether the immunity comes from vaccination, or from people having had the disease. The crucial thing is that they are immune.

* With the new coronavirus infection — called COVID-19 — as more and more people become infected, there will be more people who recover and who are then immune to future infection. 

"When about 70% of the population have been infected and recovered, the chances of outbreaks of the disease become much less because most people are resistant to infection," said Martin Hibberd, a professor of Emerging Infectious Disease at the London School of Hygiene & Tropical Medicine.

"This is called herd immunity."

WILL IT AFFECT THE COVID-19 PANDEMIC?

* With the new coronavirus outbreak, current evidence suggests that one infected person on average infects between two and three others. This means that, if no other measures are taken, herd immunity would kick in when between 50% and 70% of a population is immune.

"But it doesn't have to be –- and it won't be –- this way," said Matthew Baylis, a professor at the Institute of Infection, Veterinary and Ecological Sciences at Liverpool University.

* By reducing the number of people that one person infects — with social distancing measures such as closing schools, working from home, avoiding large gatherings and frequent hand washing — the point at which herd immunity kicks in can be lowered.

"From an epidemiological point of view, the trick is to reduce the number of people we are in contact with ... so that we can drive down the number of contacts we infect, and herd immunity starts earlier," said Baylis.

The "sweet spot" he added, is when one infected person infects, on average, one or less than one other person.