[This was circulated on 11 March 2020. Although much is still probably relevant, at that time encouraging infection amongst younger people was being discussed to achieve herd immunity. Also, interestingly, aerosol transmission was ruled out – which given my recent post was probably premature]
What is the test being used to detect CoVid19 infection?
The test used in the UK and elsewhere in Europe is to take a swab from the back of the throat and to determine if there are fragments of viral genetic material (RNA) present which are distinctive for Covid19. Thus, the test determines the virus is present and not necessarily proves active infection. This test should be able to determine the different strains of virus as it mutates in different populations.
In China, they have also been using blood tests to determine if there are antibodies to CoVid19, which prove an antibody response to a viral infection
Can the test be positive in people who do not have the infection?
This question can also be rephrased as “are there people who carry the virus and remain well but could pass on the virus to others (asymptomatic carriers)?”
There is limited data on this question as far as I am aware. Reports from China have identified asymptomatic carriers who are family members or other close contacts of infected individuals who test positive for viral RNA. These individuals are then shedding the virus without themselves ever displaying signs of illness. Interestingly, the reports thus far suggest that while many of these may not have the illness when tested, when carefully followed up, do show signs and symptoms up to 28 days afterwards. By contrast a very few still have active virus after a month but are not ill.
I am not aware of any random population studies in the areas affected eg Wuhan to see how many people with apparently no contact with an infected individual are harbouring the virus in their throat, with no illness.
What is the incubation period?
This has now been relatively well characterised and widely known. The period between being exposed to the virus and developing symptoms does vary but the median time is about 5 days (ie 50% of cases have developed by that time interval) and the large majority by 7 days. In all, 98% of those going on to have the disease have developed it by 14 days after exposure. The 28 days reported in family members above is extreme.
If someone is infected, how long are they infectious for after the symptoms start?
That is a different question to the one of incubation but important of course. The question is: assuming people have recovered from the illness, eg cough, cold and fever subsided, do they still pose a risk to others?
The Chinese data suggest that the time of maximum amount of virus shedding , ie the passing of the virus in droplets from the respiratory tract-nose and mouth, is in the early part of the infection. As the days go on the amount of virus declines. What researchers have done was to try and grow the virus from nasal and throat swabs at different time points after the illness starts. Basically by 8 days it has proved very difficult to grow the virus in anyone. Their conclusion is to assume by 10 days after symptoms have started, assuming the symptoms have stopped, people are not infectious.
However, the Chinese have also shown that fragments of the viral RNA can persist for days, perhaps for weeks after the symptoms have subsided They cannot grow the virus in culture and their conclusion though is that these genetic fragments, of themselves do not indicate that the person is still infectious. Indeed, given it has been difficult to grow the virus after 8-10 days, the persistence of the viral fragments may be of little relevance. More research will follow for sure.
How do the authorities know how many cases of coronavirus there are?
We are bombarded by numbers of new cases every day in the media: how are those numbers gathered? For the moment, the numbers are purely based on the reports submitted to the public health departments from the testing labs of the number of tests which have proven positive. For countries that have been very slow of the mark in testing (eg USA) their numbers are a woeful under-estimate. If, and when, in UK people self-isolate at home with mild symptoms and are not tested, the exact numbers will not be known. I assume that there may be sample surveys of some primary care practices to identify the numbers to make an estimate. Thus, although we should expect the numbers to rise in the next days/weeks, these numbers will under-estimate the true overall number of new cases but over-estimate the proportion of the more serve cases.
Are we now clearer about the main route of transmission?
CoVid-19 in almost every case has spread from droplets by being in close (ie 2 metres proximity) to an infected person. There is no evidence that the virus hangs around in the air (aerosol). So, for example it has been suggested that on a plane, if there is an infected person, it is only those within 2 rows who are at risk!
What is the evidence that it can be spread from hard surfaces?
In addition to droplets, the media have been keen to emphasise that where such droplets land on a hard surface they can persist. What is clearly not possible is to prove that any individual case was picked up from this route say from a particular door handle touched a week ago. Testing hard surfaces for presence of the virus is not done routinely. So the hard surface route being a source for any case or cases will I assume always remain an unproven possibility. The concern about the survival of virus on hard surfaces came from work on SARS in the laboratory which showed that on hard shiny surfaces, eg metal in the laboratory, virus could be detected sometimes up to couple of weeks after exposing the surface to the virus. Experts thus believe that the same is likely to be true for CoVid-19. Alcohol, dilute bleach and hydrogen peroxide all very effective at sterilising hard surfaces of the virus
When will this epidemic end?
This is the really interesting question and there are lots of unknowns. There are a number of possibilities:
- Successful containment: The virus can only be spread for person to person (let’s leave the issue of animal transmission) so in theory if all infected people are totally isolated then the virus could have ‘nowhere to go’. This appears to be the explanation of China’s apparent success and it is an interesting experiment to see if Italy also succeeds in their drastic measures. However, this might be a short lasting success, if infections persisted in other countries and then got re-introduced into a country where they thought it had been contained and people started mixing again, the epidemic could start over.
- Naturally die out of the virus in warmer weather One suggestion widely mentioned in the media is that (as with ‘seasonal flu’), come the warm, drier weather, the virus will struggle to survive and will then naturally die out. Epidemiologists also suggest that in summer we are less indoors and less in close contacts with other. SARS did disappear in a few months. The summer reduction theory is accepted by many although it has been suggested that CoVid19 could be with is for years and have outbreaks as with seasonal flu. Although with increasing natural immunity and a weaker virus, see below this is likely to be less serious.
- The virus will become weaker This is an interesting theory which suggests that as the epidemic proceeds the virus becomes weaker. In scientific terms the theory is that viruses mutate continuously during an epidemic. Further as the virus mutates, it becomes less virulent (causing less harm). The reason put forward is that for the virus to survive it needs to be passed on from human to human. Thus, those humans who have the more serious disease and are isolated in hospitals or die, are less likely to pass their version virus on, and there will be selective transmission of those mutations associated with milder disease. This is speculative but accords with data from other viral outbreaks
- Vaccine: A successful vaccine would easily terminate the epidemic but as I stated earlier, that even though clinical trials have started, it won’t be available for widespread us, I suspect until 2021
- Herd immunity: From discussion with colleagues, but not seen in writing, is that we should be prepared to allow (say) most ‘younger’ healthy people go about normal business, catch a mild disease and then become immune. Once the proportion of the population level with natural immunity reaches a certain level, then the non-immune people in the population would be too far spaced for an epidemic to take hold. Intriguing idea, but probably not politically acceptable and, or too risky as cannot assume only the healthy will contact the infection and, in terms of the numbers of complications, health services would be unable to cope
- Can you get it from your pet?
To finish on a lighter note, there has been a single case reported from China of transmission from patient to their dog. Most commentators have dismissed the significance of this
Making sense of CoVid-19 with Silman 