If we intend to prevent new outbreaks of COVID-19, we must study the transmission of SARS-CoV-2. Knowing the contagion routes, the transmission capacity in different population groups, the places related to the greatest risk of exposure, as well as unraveling how local transmission events occur, is key. Both to understand how the virus behaves and to develop interventions aimed at controlling its spread.
Below we summarize what we know today about how the virus is transmitted, trying to answer the main questions and emphasizing issues that arouse special interest, such as transmission by asymptomatic people or in children.
Superdispersers: individuals or events?
Unlike other infectious diseases, the superdispersion event is common and could be behind the initial success of the virus. What is superdispersion? The term refers to the fact that a few infected are capable of infecting a large number of people, while the vast majority do not transmit the virus. In the case of the coronavirus, it is estimated that 10% of those infected contribute to 80% of the transmissions.
Although it is a word with which we are increasingly familiar, it is important to ask ourselves: who is the super disperser, the event or the individual? Identifying an individual as a super-disperser can lead us, prejudiced, to suppose that they either act deliberately or lead a life with risky practices.
But the truth is that we do not have evidence on common characteristics of potential super-scatters, since in fact many transmitters are asymptomatic or presymptomatic . However, we do have evidence that massive contagion occurs under common circumstances. We refer, above all, to the presence of infected people in closed spaces and in continuous contact with other people. Hence, it is more correct to speak of super scattering events.
Currently, given the difficulties in detecting all the people capable of transmitting that we will develop later, an effective preventive strategy is to avoid risky situations with specific confinement and de-escalation measures.
Where is the virus transmitted?
Generally, transmission takes place mainly in confined spaces where there are close contacts and large numbers of people. Any activity that is associated with rapid breathing can also pose a risk for transmitting the virus. For example, in a gym there was an outbreak among those attending a Zumba class, but not among those attending yoga and Pilates, even though they shared space.
Reviewing a compilation of the best known outbreaks, we can see that most occur in indoor places of a very diverse nature (from religious centers to cruise ships and student residences). Places where there is usually a high agglomeration of people or with a shared use of common areas by many people.
Can asymptomatic individuals transmit the virus?
It is important to point out that in this category the real asymptomatic are mixed with the presymptomatic ones, that is, those who transmit just before presenting symptoms. In any case, it is estimated that they represent around 20-40% of those infected, depending on the population segment. And it has been documented that they are transmitters . In fact, they are considered a very important factor in the ‘success’ of SARS-CoV-2.
In the case of SARS, the transmission of the virus was associated with specific symptoms. For this reason, early diagnosis made it possible to isolate cases before they were transmitted to healthy individuals. The initial response of Spain and many other countries to COVID-19 was based on this previous experience: “passive” identification of cases. That is, wait for infected individuals to present specific symptoms and go to health services to make an early diagnosis.
However, with COVID-19 this strategy is not valid, since there is a presymptomatic period of about several days and there is a high percentage of asymptomatic cases. All of them are capable of transmitting before the health system detects them. Hence the importance of adopting an active search for cases during de-escalation. In other words, propose a strategy based on the triad: test, follow-up and isolation ( test, trace, isolate ).
This method consists of diagnosing any person with some minimal symptoms, isolating them, finding their contacts, diagnosing them, isolating them… And thus stopping the chains of transmission before the number of secondary cases is so great that they cannot be traced. The latter is what happened in the first wave of the pandemic.
However, this strategy is only valid as long as the number of infections is low. Each case has between 5 and 10 contacts to study (at least). This causes the number of total contacts to rapidly increase to uncontrollable dimensions . Therefore, individual citizen action of social distancing and adoption of protection measures is essential to keep the number of infections low.
In this sense, the first robust evidence on the effectiveness of social distance and masks has recently been published.
When is the virus transmitted?
Related to the previous point, the question arises as to when an infected person is capable of transmitting the virus. A recent study indicates that a significant part of infected patients who have transmitted the virus (44% of the events analyzed) have done so during the presymptomatic period (up to 2-3 days before showing the first symptoms). In fact, this study shows that the highest infective capacity is reached just before showing the first symptoms. In other words, these individuals are capable of transmitting the virus before the healthcare system has detected them.
Does age affect the ability to transmit the virus?
The transmission capacity of SARS-CoV-2 seems the same in the different population segments, except those under ten years of age. The strongest hypotheses suggest that the contagion capacity depends on the viral load, although more scientific evidence is needed . Assuming this is so, we could predict whether there are differences in transmission rate between different age groups by analyzing their viral load.
In a recent study in Germany of about 4,000 COVID-19 patients, the viral load of the participants was measured, concluding that there are no significant differences in viral load based on age. A limitation of the study is that it has only been carried out with hospitalized patients, excluding individuals with mild or asymptomatic symptoms.
Are children at greater risk of transmitting the virus?
This is one of the most contentious questions. An affirmative answer could lead to the prolonged closure of schools, with all the far-reaching social consequences that this entails, such as conciliation problems, the setback in equality issues and in the teaching of children … Therefore, it is of the utmost importance to determine if school and children are transmission vectors.
In certain diseases, such as influenza, prepubertal children are clearly facilitators of transmission. But when we talk about COVID-19 we find ourselves in a different scenario, in which we have evidence both for and against this fact.
The previous study showed us that children have the same viral load as adults. However, the children included are those with quite severe symptoms, and therefore may not be a representative sample of the real scenario. This low prevalence, on the other hand, could be explained by the fact that the expression of angiotensin-converting enzyme II (ACE2) in the nasal epithelium is lower in children, although a more extensive cohort is required to be able to affirm this.
In contact studies from several countries, children have not been found to be index cases of outbreaks. This means that in the outbreaks studied, no child was found to be the start of the chain of infection. But the limited capacity of the system to detect these cases must be taken into account, since children are more asymptomatic than adults. And while it is true that there have been some outbreaks in schools, it is very difficult to determine whether the index case was a schoolboy or an adult.
A recent model suggests that closing schools could be an effective measure to reduce the peak incidence by 40-60% and thus slow down the epidemic. They argue that, although children are less susceptible than adults, they compensate because they have many more contacts and risky practices that make them more exposed and more likely to transmit.
For its part, a meta-analysis does identify children as individuals less susceptible to infection, but the data on their relationship with transmission are inconclusive.
In short, more robust investigations are still needed that can support decision-making in matters that imply a high social impact such as the closure of schools.
Are people who have tested positive after being discharged infectious?
Since the beginning of the pandemic, cases of patients who obtain a positive result after their complete recovery have been reported. It has been seen as common in up to 30% of patients. Although it is alarming that some patients, days or weeks after being discharged from hospital, test positive in the PCR test, it is a fact that we must handle with caution.
What those cases represented was unclear in the early days of the epidemic. Was it reinfection? Or perhaps a poorly resolved infection with a reservoir where the virus hides? Or maybe detection of non-infective genetic material?
To date, the best evidence comes from South Korea, where 285 of these patients and their contacts were followed. After the epidemiological study and the testing of the contacts, it was concluded that none of those analyzed was positive, thus indicating that patients with a persistent positive PCR result are not contagious, and the analysis of the detected virus remains confirmed that they had lost their infective capacity.
Are there tools to track the transmission?
Genomic epidemiology makes it possible to follow the transmission and movement of the virus at any scale by studying the sequence of its genome. In Spain, different groups are working on genomic epidemiology of the virus, including a consortium, SeqCOVID Spain , of more than 40 hospitals and research centers that intend to sequence between fifteen and twenty thousand samples from as many patients. By itself, the genomic sequence does not have sufficient resolution to analyze the detail of the outbreaks. But when we combine it with epidemiological information, it becomes a very powerful tool.
An example is the study carried out in a hospital in South Africa to solve a nosocomial outbreak that involved 119 individuals between patients and healthcare personnel. What is truly important about this type of investigation is that they are capable of providing valuable information to the Public Health authorities, so that they can adopt measures to control the transmission of the virus.
Authors: Iñaki Comas Espadas, senior scientist, Tuberculosis Genomics Unit, Institute of Biomedicine of Valencia (IBV – CSIC), Álvaro Chiner Oms, post-doctoral researcher (bioinformatician) at the Tuberculosis Genomics Unit, Institute of Biomedicine of Valencia (IBV – CSIC); Ana María García Marín, PhD student, Valencia Institute of Biomedicine (IBV – CSIC) and Fernando González Candelas, Professor of Genetics and head of the Joint Research Unit “Infection and Public Health” FISABIO-Universitat de València I2SysBio. CIBER Epidemiology and Public Health, University of Valencia.
This article was originally published on The Conversation. Read the original.
