Severe Acute Respiratory Syndrome (SARS) refers to a group of respiratory illnesses caused by related coronaviruses. The acronym is most commonly associated with the 2003 outbreak of SARS-CoV-1 and the subsequent global pandemic caused by SARS-CoV-2, which leads to the disease COVID-19. Understanding the biology and movement of these viruses is central to developing effective strategies for prevention and mitigation.
Viral Classification and Zoonotic Origin
The viruses that cause SARS are members of the Coronaviridae family, large RNA viruses characterized by spike proteins on their surface that give them a crown-like appearance. These viruses belong to the Betacoronavirus genus and specifically the subgenus Sarbecovirus. The spike protein serves a function by binding to the Angiotensin-Converting Enzyme 2 (ACE2) receptor on human cells to gain entry.
SARS viruses are of zoonotic origin, meaning they jump from an animal reservoir to humans. The natural reservoir is primarily horseshoe bats, which can harbor them without showing signs of illness. Before reaching humans, the virus often passes through an intermediate animal host where it may mutate further. For SARS-CoV-1, the intermediate host was likely the civet cat, while the precise intermediate host for SARS-CoV-2 remains an area of ongoing study, with evidence pointing toward animals like pangolins.
Core Mechanisms of Transmission
SARS viruses primarily spread through the release of respiratory particles expelled by an infected person during activities like breathing, talking, coughing, or sneezing. This process is categorized into two mechanisms: larger droplet transmission and fine aerosol transmission.
Droplets are larger respiratory particles that fall quickly to surfaces or mucous membranes within a short range, typically a few feet. Infection occurs when these particles land directly on the eyes, nose, or mouth of a susceptible person nearby.
Aerosols are much smaller particles that remain suspended in the air for longer periods and can travel greater distances. These particles can be inhaled deeply into the respiratory tract, making aerosol inhalation a significant route for infection, particularly in poorly ventilated settings.
Transmission can also occur indirectly through contaminated surfaces, known as fomites, when a person touches an infectious surface and then touches their own face.
Distinct Differences Between Major Outbreaks
The two most significant diseases caused by SARS viruses, SARS (SARS-CoV-1) and COVID-19 (SARS-CoV-2), exhibit differences in their epidemiological patterns despite their genetic similarity.
The original SARS-CoV-1 outbreak of 2003 was characterized by a high case fatality rate, estimated to be around 9.6% to 11%. However, the virus was mainly transmitted by individuals who were already symptomatic and ill, which made it easier for public health officials to identify and isolate cases.
In contrast, SARS-CoV-2 is characterized by a lower overall fatality rate but higher transmissibility, leading to a global pandemic scale. A defining feature of SARS-CoV-2 is the high degree of presymptomatic and asymptomatic spread.
People can efficiently transmit the virus days before they feel sick, or even if they never develop symptoms, allowing the virus to circulate widely and silently within communities. The viral load also appears higher in the upper respiratory tract earlier in the infection course for SARS-CoV-2, which facilitates expulsion and transmission through normal breathing.
Strategies to Limit Community Spread
Public health efforts to interrupt the spread of SARS viruses focus on mitigating the risk associated with respiratory particle transmission.
Source control, primarily through the use of face masks, works by blocking the expulsion of virus-containing droplets and aerosols from an infected person, protecting others. Physical distancing measures reduce the likelihood of close-range droplet transmission and high-concentration aerosol exposure.
Improving indoor air quality is a targeted intervention against aerosol spread, involving the use of ventilation systems to increase the exchange rate of fresh air and filtration devices to remove infectious particles. Consistent hand hygiene, using soap and water or alcohol-based sanitizers, prevents infection via fomites and accidental self-inoculation.
The development and widespread deployment of effective vaccines train the immune system to recognize and neutralize the virus, decreasing the rate of infection and severe illness.