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.
- Preventing or reducing risks of exposure to the virus that causes the illness.
- 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.
- 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.
- The lower the absolute volume of circulating virus the lower the risk of viral mutation.
- Slowing the evolution of the epidemic buys time in order to:
- develop tests to help us better understand the nature of the infection and the optimal control methods
- develop treatments to optimally manage the illness
- develop a vaccine which is ultimately the most effective method of controlling epidemics of infectious disease.
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 likelihood of an epidemic taking hold.
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
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.
Most importantly we can be vaccinated against Covid. We now know that vaccines are very safe and effective.
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 for each illness. 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 is likely to grow. Infections with higher R0 values are more likely to spread. The new variants of Covid are more transmissible which means that they have higherR0 numbers. This means that public health measures which were adequate to control the first waves of Covid would not be as effective for a newer variant and more aggressive controls or lockdowns may be necessary.
Population measures such as isolation of infected individuals, masks, hand washing, school closures, 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. Ultimately achieving high population uptake of an effective vaccination is the most important factor in allowing Hong Kong to reduce existing public health controls and returning to normal. Although Zero covid would be the ideal situation, unfortunately it is impossible to achieve in the long term and vaccination is the only viable solution. We have explained this concept in more detail in another article.