Publication - Research and analysis

Coronavirus (COVID-19): framework for decision making - assessing the four harms

Sets out the four harms process for assessment used to establish when coronavirus restrictions could be safely lifted after lockdown and the scientific evidence underpinning the decisions.

Coronavirus (COVID-19): framework for decision making - assessing the four harms
The science behind COVID-19

The science behind COVID-19

A summary of the science behind Coronavirus sets the scene for an understanding of the decisions involved in the assessment process.

Coronavirus, like all viruses, is a tiny infectious pathogen. Viruses typically contain genetic information surrounded by a protein coat and viruses use their protein coat to latch on to a host cell and insert genetic material inside. Viruses can only replicate using the cells of a living organism[18]. Different viruses can infect animals, plants and even bacteria and always use the hosts' cells to replicate themselves. Viruses are not bacteria so cannot be treated with antibiotics. Sometimes viruses can cause no symptoms and are therefore no threat to their host, while other viruses can lead to disease which can be mild or severe depending on the pathogen, and the characteristics of the host.

Coronaviruses are a group of viruses that have a crown-like (corona) appearance when viewed under a microscope. Coronaviruses are a large family of viruses which can cause illnesses in animals and humans. Some coronaviruses can cause the common cold, with mild symptoms that only last a short time. Asymptomatic infections from coronaviruses have also been described[19]. However, two other human coronaviruses, Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS), have been known to cause severe symptoms and even death. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the name of the virus that causes the COronaVIrus Disease (COVID-19) so named because it first occurred in 2019. Both the virus and the disease were unknown prior to the outbreak in Wuhan, China in late 2019 and it is possible that the initial source of the virus was from an animal[20]. COVID-19 spread rapidly worldwide and became a global pandemic because it affected a large number of people in a large number of countries. COVID-19 is a respiratory disease: most people infected will experience mild to moderate respiratory illness and recover without requiring special treatment. Older people, and those with underlying medical problems like cardiovascular disease, diabetes, chronic respiratory disease, and cancer are more likely to develop serious illness.[21]


COVID-19 is thought to be spread predominantly through droplets of saliva or discharge from the nose. Droplets are particles that carry the virus and will normally settle out of the air in less than 5 minutes. These typically deposit on people and surfaces less than 2 metres from the source. These droplets can be passed to others through direct contact, through close proximity or through transfer on surfaces. COVID-19 is also spread through aerosols, tiny respiratory particles which can remain in the air for several minutes. Aerosols containing the virus can potentially be inhaled by another person, leading to infection[22]. This is most likely to occur when people are in close proximity to each other. Aerosols can increase the risk of transmission in poorly ventilated indoor spaces, so spending a lot of time with other people in a poorly ventilated space is not recommended[23]. Good ventilation will dilute the air and remove the virus but there is currently no evidence for aerosol spread between rooms or over long distances outdoors.

Transmission is likely to occur from a combination of droplets, aerosols and through direct contact with surfaces. Most transmission is believed to happen when people are in close proximity to each other (under 2 m distance)[24].This means that physical distancing, good hand hygiene and cleaning of surfaces remain very important infection control measures.

Both asymptomatic and pre-symptomatic transmission of COVID-19 is now known to occur. This means that people who have no symptoms can still be infectious to others. For this reason, it is advisable to wear face coverings in addition to other measures to help prevent transmission. This is particularly important in indoor environments with poor ventilation or where social distancing is not possible at all times. Face coverings are likely to provide some benefit in reducing the risk of aerosol transmission[25]. Face coverings will reduce the dispersion of respiratory droplets and small aerosols that carry the virus into the air from an infected person. They also provide a small amount of protection for the wearer against exposure to droplets but less protection against small aerosols.


The primary and most important mode of transmission for COVID-19 is through close contact from person-to-person. Based on data from lab studies on COVID-19 and what we know about similar respiratory diseases, it may be possible that a person can get COVID-19 by touching a surface or object that has the virus on it and then touching their own mouth, nose, or possibly their eyes. This, however, is not thought to be the main way the virus spreads. Several laboratory-based studies have been carried out looking at the survival of the virus on different surfaces. Studies have shown that the COVID-19 virus can survive for up to 72 hours on plastic and stainless steel, less than 4 hours on copper and less than 24 hours on cardboard[26]. Some studies suggest that the virus could survive a number of days, depending on the surface, but this usually involved using a large dosage of virus in a laboratory environment[27]. The most important thing to know about coronavirus on surfaces is that they can easily be cleaned with common household disinfectants that will kill the virus. Washing your hands with soap and water or using alcohol-based hand rub kills viruses that may be on your hands.

Duration and Proximity

The biggest risks are associated with close contact with an infected individual for a prolonged period of time[28]. This could be within 2 metres or within the room of someone carrying the virus. Close contact can also include direct contact with infectious secretions, this could be from sharing eating or drinking utensils, or direct contact with an infectious person, such as hand shaking, hugging and kissing. Close contact does not include activities such as walking past a person or briefly sitting across from someone in a waiting room or office. A recent article noted that:[29]

Contact tracing studies provide early evidence that sustained close contact drives the majority of infections and clusters. For instance, living with the case, family/friend gatherings, dining, or travelling on public transport were found to have a higher risk for transmission than market shopping or brief community encounters.


Each setting brings different risks. Some settings are particularly high risk, such as care homes, as these usually have many people living under one roof, in a situation where social distancing is difficult. The elderly are among the most vulnerable and those living in care homes often require personal care. Healthcare settings will also have to deal with vulnerable groups due to illness, whether treating COVID-19 cases or for other health-related admissions.

People spend most of their time either in the home or at work. The size of a residential home, number of rooms and number of occupants will affect the ability of an occupant to isolate in the home. More people in the home will increase the potential risk of transmission to others, and this will be exacerbated if there is insufficient space for people to isolate, due to sharing of bedrooms and living spaces[30].

Work settings are also important as colleagues will often spend several hours of the day together. In these settings, it is important to consider the space available, shared facilities and ventilation. Social settings are also important, whether this is hospitality, retail, visiting friends and family[31]. Much consideration has recently been given to higher educational settings and risks associated with these.

Assessment of the risk of these settings takes in to account several factors including residential accommodation, class room and lecture space, vocational teaching and sports and social clubs associated with the educational facility[32]. The demographics of those attending each setting is also taken in to account.

A recent publication from the BMJ provides a good summary of setting risk:[33]

Most transmission occurs through close range contact (15 minutes face to face and within 2 m) and spread is especially efficient within households and through gatherings of family and friends. Household secondary attack rates (the proportion of susceptible individuals who become infected within a group of susceptible contacts with a primary case) range from 4% to 35%. Sleeping in the same room as, or being a spouse of an infected individual increases the risk of infection, but isolation of the infected person away from the family is related to lower risk of infection. Other activities identified as high risk include dining in close proximity with the infected person, sharing food, and taking part in group activities. The risk of infection substantially increases in enclosed environments compared with outdoor settings. Aerosol transmission can still factor during prolonged stay in crowded, poorly ventilated indoor settings (meaning transmission could occur at a distance >2 m).