What are social distancing measures and how do they work?

Why are the playgrounds closed? Why do restaurants close at 6 pm?

Written by Dr David Owens

It is important to appreciate that in epidemics we can only protect ourselves, our families and our children in one of two ways.

1. 1. Preventing or reducing risks of exposure to the virus that causes the illness.
   2. Developing immunity against the disease.

We can only develop immunity in one of two ways. We can either be infected, which carries the risk of illness, complications and even death or we can be vaccinated.

The public health measures in any epidemic are aimed at slowing down both the rate of progression and ultimate size of the epidemic.

1. 1. The lower the epidemic size (number of people infected) the lower the number of people who will become ill and the lower the total mortality.
   2. The lower the absolute volume of circulating virus the lower the risk of viral mutation.
   3. Slowing the evolution of the epidemic buys time in order to:
      
      • test and isolate individuals and test/or isolate contacts to help us to break the transmission chains
      • maximise vaccine coverage, which is ultimately the most effective method of controlling epidemics of infectious disease
      • protect the health system from being overloaded by ‘flattening the wave’ of infections

In addition, the measures above may be expected to reduce other coexisting infectious illnesses (Influenza, common colds etc.). This can boost population immunity which reduces the population impact of an epidemic.

How do we prevent infections from spreading?

All infectious illness is ultimately decided by two factors:

1. Exposure risk

If we are never exposed to an infectious illness, we will not catch it. We can reduce risk of exposure by:

• Isolating infectious individuals and tracing their contacts: Test, trace, isolate
• Quarantine both of contacts and at border entry
• Closing locations where infections tend to spread, bars, clubs, if needed schools
• Wearing masks
• Reducing public mixing (crowds and travel): social distancing
• Hand washing

All the above measures work by reducing the exposure risk. They are all referred to as non-pharmacological interventions (NPI).

2. Host response

Most importantly we can be vaccinated against Covid. We know that vaccines are very safe and effective. They significantly reduce the severity of illness and the mortality rate.

Even if we are exposed to an illness we can reduce our likelihood of having a severe response by having a good immune system. This is important both for individuals (diet, sleep, managing stress, exercise etc.) but also for population immunity.

What influences the size of an epidemic?

All infectious illness is influenced by the following factors:

• Mode of Spread
• Incubation Period
• Infectivity in the Incubation
• Individual and Population Immunity

These factors ultimately lead to a basic reproduction number R⁰. This number is a measure of on average how many people are infected by an individual with the specific disease. If this number is <1 the epidemic will die (this means that on average each infected person infects slightly less than one other person). If this number is >1 the epidemic will grow. Infections with higher R⁰ values are more likely to spread. Values of R⁰ are not fixed. They vary dependent on external factors such as population density, temperature and seasonal change. The new variants of Covid, especially Omicron are more transmissible which means that they have higher R⁰ numbers. This means that public health measures which were adequate to control the first waves of Covid will not be as effective for Omicron and more aggressive controls may be necessary to mitigate the size of the epidemic wave.

Population measures such as isolation of infected individuals, masks, school closures and social distancing all work by reducing the exposure risk with the intention of reducing the effective transmission below 1 but regardless as much as possible in order to give the best possible chance for the epidemic to be controlled. Some of the policies may seem irrational and some undoubtedly are. Media reports questioning the rationale of evening dining restrictions but not those earlier in the day, or regulating face-to-face contact in primary schools yet not secondary schools, reflects a lack of effective communication around public health regulations. Ultimately, social distancing measures are about reducing probability events. We don’t close beaches because of the risks on the beaches but because of the increasing touchpoints on public transport and in changing rooms. Of course every intervention has both a risk/benefit and also unintended consequences. In the case of closing beaches we may drive people into shopping malls increasing risk. Individual social distancing policies are judgement calls which balance the risks and benefits to the population and are aimed predominantly at reducing the absolute number of mask off person-to-person contacts within the city whilst maintaining as much economic and social function as possible.

Ultimately achieving high population immunity by vaccination especially from the most vulnerable down is the most important factor in allowing Hong Kong to reduce existing public health controls and return to normal. Zero covid is not a sustainable long term solution and vaccination is the key tool in transitioning to living with Covid.