Dr Iain Haysom, our Senior Lecturer in Food Safety and Microbiology, shares the latest research and describes how the coronavirus spreads.
How quickly the world can change and become a very different place from the one we thought we knew.
In December 2019, reports started to filter through from China of a new pneumonia-like disease; now, just four months later, the number of cases globally has exceeded three and a half million and is rising daily. 187 out of 194 countries have reported cases. Many countries are in lockdown in an effort to slow the spread of the disease, and epidemiologists, microbiologists and clinical researchers are working around the clock to try to understand this new threat to our health and offer tests and treatments.
How have we gone from most people having never heard of coronavirus to a situation where discussions of epidemiological statistics, flattening the curve and social distancing have become commonplace?
What is Covid-19?
The coronavirus family of viruses was first identified in the late 1960s, and these viruses are reasonably well understood. While most coronaviruses pose little in the way of health risks to humans, viruses in this family are thought to cause around 20-40% of common colds.
A couple of strains are able to cause much more serious illness, such as the one responsible for the SARS (Severe Acute Respiratory Syndrome) virus outbreak in 2003 and the one responsible for the MERS (Middle East Respiratory Syndrome) virus outbreak in 2012. This family of viruses seems to have its natural reservoir in bats but has been able to mutate, enabling it to infect other animals including humans. Initial reports suggest the virus responsible for the current pandemic jumped from bats to pangolins, and then from pangolins to humans.
How viruses spread
Viruses are fascinating microorganisms. Most consist of little more than a protein coat which contains nucleic acid, either DNA or RNA. Some viruses, such as the coronavirus, additionally have an outer membrane of fats and proteins.
In order to replicate and increase in number, a virus has to infect a living cell. In the wider environment, such as on surfaces or door handles, viruses are completely inert, incapable of doing anything. They have no metabolism, no need for food or water and cannot increase in number.
Once they are able to infect a living cell however, the situation becomes very different. Once the viral DNA or RNA gets into the cell, it essentially hijacks the metabolism of that cell and causes it to manufacture all the components required to build new copies of the virus. These new viruses are then released from the host cell, sometimes causing its death in the process, and are then free to infect new host cells. The irony of being ill with a viral infection is that it is your own body’s cells that are manufacturing the new viruses that are making you ill.
The new strain of virus responsible for the current pandemic has been called SARS-CoV2 and the resulting disease is known as Covid-19.
Although this is a previously unknown strain of virus, the pace of research and global cooperation to tackle it has been heartening.
Despite first being reported in December 2019, the full genome of this new virus was published in January 2020, enabling laboratories around the world to begin work on tests to detect the virus, and also to understand it better. Great strides have already been made in understanding the epidemiology and pathogenesis of this virus which are informing global efforts to contain it.
Why has this disease spread so rapidly?
Viruses need to be able to attach to a specific receptor on the surface of a host cell in order to cause infection. For SARS-CoV2 that receptor is a protein called ACE-2, which is found on the surface of cells in the upper and lower respiratory tract, including cells within the alveoli of the lungs. The virus attaches to these receptors more strongly than previous strains of coronavirus, which probably means we need to inhale fewer viruses to cause an infection.
It might be that during upper respiratory tract infection, the symptoms are mild or not recognised and during this period, the infected individual can infect others. As the viruses progress down to the lower respiratory tract and lungs, the symptoms become more apparent and more severe.
Lower respiratory tract infections can cause damage to the lung lining causing the lungs to fill up with debris from dead cells and fluid. The virus also triggers a number of other responses which put additional pressure on the lungs, making it hard to breathe and difficult for the body to get oxygen out of the lungs and into the bloodstream.
As SARS-CoV2 is a respiratory virus, it is spread in droplets emanating from someone with the disease, usually when coughing or sneezing, but potentially also when talking normally. These droplets are large enough that they won’t remain airborne for long but will settle onto the floor or surfaces, generally within three hours after expulsion.
Individuals become infected either by directly breathing in infected droplets from the air before they settle, or by touching an infected surface and then transferring viruses from that surface to their mouth or nose or eyes. Research into the spread of the virus suggests that most of the infected droplets will fall to the ground within two metres of an infected individual.
Other studies looking at how long the virus remains stable on surfaces indicate it may be just a few hours on metal, up to 24 hours on cardboard and up to two or three days on plastic.
Knowledge of these issues enables scientists and public health officials to offer clear, science-based guidance about how to reduce the risk of catching and spreading this virus.
Impact and death rate
The fatality rate of this coronavirus is much lower than both SARS and MERS, however because it is spreading much faster than the virus in those outbreaks, the total death rate is already higher than both those previous outbreaks combined.
A major contributing factor to this is the ease by which this virus seems to be able to spread from person to person, and also the length of time after infection when the individual still feels well but can still spread the virus to others. On top of all this, most people are immunologically naïve, meaning they have no immunity to the pathogen. All of these issues point to why this strain of the virus is proving to be such an effective pathogen and had spread across almost the entire globe within three months.
Person to person spread would appear to be the main route of transmission, with most of this currently occurring in the community rather than in health care facilities.
The term for the number of people each infected case can spread the disease to is known as the reproductive number, or R0, and for SARS-CoV2 current estimates put this somewhere between two and three. This means that every case can spread the infection to two or three people. This doesn't sound too high, but is approximately twice as high as the R0 for flu, to which this disease is often compared. What this means in practice is that a single case could spread the infection to three new people, and those three could each spread the disease to three more people, and so on.
After ten cycles of this, there will be over 59,000 cases of illness arising from that initial single case. While that is a worst case scenario and assumes each case will actually infect three more, it does show how rapidly case numbers can increase. It is this rapid increase in the number of cases that can lead to health systems becoming overwhelmed.
For this reason, social distancing, reducing as far as possible direct contact with other people, and trying to maintain a two metre gap when we do have to interact, is key.
Positive action
Being conscious of how many times we touch our face, especially around the mouth, nose and eyes is also important as that can transmit viruses on our hands to our respiratory system. There is little rigorous research into how often we touch our faces, but a figure of 10-20 times an hour seems realistic. It is such an involuntary habit that it is hard to stop, but a recent article in the British Medical Journal recommended that rather than telling yourself not to touch your face, you give yourself a positive action to enforce, such as keeping your hands below shoulder level.
Cleaning and hygiene is also of fundamental importance, and the good news is that ordinary soap is highly effective. Coronaviruses have an outer membrane primarily made of fats, with some proteins. This outer membrane dissolves in soap, which renders the virus incapable of causing infections, effectively killing the virus. Special hand-sanitisers and antimicrobial washes are not required, although a hand sanitiser with at least a 60% alcohol content is useful in situations where you cannot wash your hands, as that level of alcohol is also fatal for the virus.
If you cannot wash your hands, try not to touch your face until you are able to get to a sink with soap and water. A twenty second rigorous hand wash using soap and ensuring you wash all over your hands and wrists is sufficient to remove virus from your hands.
Global and local responses
The rapid spread of this disease has shown just how connected we are as individuals and as a society to everyone on the planet.
In terms of responding to this pandemic, attention has rightly focused on the frontline medical staff who are treating those who become unwell, but it is going to take the collective efforts of all of us as members of a global society to help halt the progress of this disease.
Every single one of us has a role to play in tackling this virus.
Without a current vaccine or antiviral treatment, we all have a responsibility to reduce our risk of becoming ill and of spreading the virus to others. The current measures are all about flattening the epidemic curve. This means slowing down the rate of spread of the epidemic through the UK to ensure the health service can cope with the cases that do arise.
Dramatically reducing the opportunities for the virus to spread from person to person means the rate of new cases is slowed down. Practising good hygiene means we reduce the risk of people picking up viruses from surfaces. By doing this, you are not just protecting yourself, you are protecting your friends, family and the rest of society.