What is herd immunity? 

When a high percentage of the population is vaccinated, it is difficult forinfectious diseases to spread, because there are not many people who can be infected.

For example, if someone with measles is surrounded by people who are vaccinated against measles, the disease cannot easily be passed on to anyone, and it will quickly disappear again.

This is called ‘herd immunity’,'community immunity' or 'herd protection', and it gives protection to vulnerable people such as new-born babies, elderly people and those who are too sick to be vaccinated.

Herd immunity does not protect against all vaccine-preventable diseases. 

The best example of this is tetanus (破伤风), which is caught from bacteria in the environment, not from other people who have the disease.

No matter how many people around you are vaccinated against tetanus, it will not protect you from tetanus.

How does herd immunity work?Herd immunity only works if most people in the population are vaccinated(for example, 19 out of every 20 people need to be vaccinated against measles to protect people who are not vaccinated).If people are not vaccinated, herd immunity is not guaranteed to protect them. Unlike vaccination, herd immunity does not give a high level of individual protection, and so it is not a good alternative to getting vaccinated.

Will the UK's coronavirus strategy work?

There are many differnt opinions on this. Herd immunity, as a tactic in fighting a pandemic for which there is no vaccine, is novel – and some say alarming (the guardian, 2020).The same article says this strategy relies on people getting the disease – in this case Covid-19 –and becoming immune as a result. Generally it is thought that those who recover will be immune, at least for now, so they won’t get it twice.

An article from the Telegraph, says the measures introduced in Wuhan are unprecedented and will have had a huge social and psychological toll.
Although they have reduced transmission by around 55 percent in 2 weeks, there is evidence that the vast majority of the population is still susceptible. As soon as control measures are lifted, there is the risk of new introduced cases, maybe another outbreak. 
They argue that it's easy to say let's just do what Wuhan did, but the measures have involved a change to daily life that really has been unimaginable in scale and impact. It cannot sustain indefinitely. 

In an article on 15th March, ‘The UK backs away from “herd immunity” coronavirus proposal amid blowback’The author says the United Kingdom is previewing a new self-isolation plan for all people over 70 to address the spread of the novel coronavirus in the country.“in the coming weeks,” people over the age of 70 will be told to self-isolate.''This stands in contrast with World Health Organization guidelines, which recommend that everyone, regardless of age, practice social distancing.

The herd immunity controversyThe argument behind achieving herd immunity is that doing so would minimize the risk of a coronavirus resurgence similar to the second wave of the Spanish Flu of 1918.In a successful herd immunity scenario, a country’s population — the herd — would become infected with the virus, recover, and then have an immunity to it, making them resistant to reinfection.For this to work in his country, according to Vallance, the virus would need to spread to about 60 percent of the UK.That reality led to more than 200 scientists and medical professionals pushing back against the herd immunity strategy in an open letter Saturday.These experts argued that herd immunity does not “seem a viable option” because it could overwhelm the UK’s National Health Service with very sick patients.Instead, they called for strict social distancing measures of a more serious variety than the government has currently recommended.

Reference:University of Oxford, available: https://vk.ovg.ox.ac.uk/vk/herd-immunity (accessed: 15 Mar. 2020)Boseley, S. (2020) ‘Herd immunity: will the UK's coronavirusstrategy work?’ available:https://www.theguardian.com/world/2020/mar/13/herd-immunity-will-the-uks-coronavirus-strategy-work (accessed: 15 Mar. 2020)Wagtendonk, S. (2020) 'The Uk backs away from heard immunity coronavirus proposal amid blowback'https://www.vox.com/world/2020/3/15/21180414/coronavirus-uk-herd-immunity-vallance-johnson