I have heard coronavirus are a family of viruses and I was also wondering if people testing positive have another coronavirus.

They might but it's unlikely the test would falsely identify another known coronavirus as it deliberately looks for genes that are found only in SARS2 (although some of them are also found in SARS1, the closest known human-infecting relative, but that seems to have died out).

But if you are infected with some unknown close relative of SARS2 the effect is the same: try not to infect anyone else, get plenty of rest, and seek medical attention if you have difficulty breathing.

There's a good graphical calculator here that uses "Bayes Formula" to work out the probability you have Covid if you have a positive test based on prevalence, test sensitivity and specificity:

https://www.bmj.com/content/369/bmj.m1808

The box on the top left defaults to 80%. This is hopelessly unrealistic as the number of people infected with SARS-2 at any one time will only ever get that high at the peak of a very rapid outbreak in a small crowded environment (a prison or care home perhaps).

The lowest you can set that to is 1%, which means a positive result means you have a 16% chance of actually having Covid. But the real prevalence is much lower than that-- 0.09% in the most recent ONS Covid-19 Infection Survey. But even that is so low that the 95CI (if you account for the specificity of the test, which the ONS don't appear to) starts well below zero.

I have heard coronavirus are a family of viruses and I was also wondering if people testing positive have another coronavirus.

They might but it's unlikely the test would falsely identify another known coronavirus as it deliberately looks for genes that are found only in SARS2 (although some of them are also found in SARS1, the closest known human-infecting relative, but that seems to have died out).

But if you are infected with some unknown close relative of SARS2 the effect is the same: try not to infect anyone else, get plenty of rest, and seek medical attention if you have difficulty breathing.

There's a good graphical calculator here that uses "Bayes Formula" to work out the probability you have Covid if you have a positive test based on prevalence, test sensitivity and specificity:

https://www.bmj.com/content/369/bmj.m1808

The box on the top left defaults to 80%. This is hopelessly unrealistic as the number of people infected with SARS-2 at any one time will only ever get that high at the peak of a very rapid outbreak in a small crowded environment (a prison or care home perhaps).

The lowest you can set that to is 1%, which means a positive result means you have a 16% chance of actually having Covid. But the real prevalence is much lower than that-- 0.09% in the most recent ONS Covid-19 Infection Survey. But even that is so low that the 95CI (if you account for the specificity of the test, which the ONS don't appear to) starts well below zero.

A common skeptic question is that if coronavirus is dangerous and produces severe symptoms, why would they need to test people to see if they have the illness. For instance, the symptoms of illnesses such as bubonic plague and Ebola are so horrific, you wouldn't need to test people to see if they have the illness. Do you feel this is a valid point.

A common skeptic question is that if coronavirus is dangerous and produces severe symptoms, why would they need to test people to see if they have the illness. For instance, the symptoms of illnesses such as bubonic plague and Ebola are so horrific, you wouldn't need to test people to see if they have the illness. Do you feel this is a valid point.

Yes this is a valid point. For some things you want to do a test in order to decide on a treatment. But at the height of the pandemic, we either didn't have tests (in the UK) or the results took about a week to come back. Either is useless if you need to treat someone who is having trouble breathing. Doctors got pretty good at spotting Covid from lung X-rays and were using that for diagnosis.

So what is the point of testing? It can be useful if you are trying to contain an epidemic before it has spread. If you catch cases early, trace contacts, and quarantine anyone who tests positive, even if asymptomatic, it is possible to keep the infection rate in your country very low without having a lockdown. Iceland achieved this. But it's not clear how much more you achieve with this than people just voluntarily quarantining themselves when they have symptoms of any kind of cold or flu, and using simple devices like IR thermometers. Although asymptomatic people and presymptomatic people can spread the virus in theory, it's likely only to be a small amount, so it makes more sense to concentrate your efforts on the symptomatic. This is far easier to do, requires no technology, and has almost no collateral damage to society or health.

The other thing testing lets you do, which is actually useful, is just to know if the pandemic has reached your country yet at all. If not then it might be worth being very careful at your borders and quarantining people coming in. But SARS2 had got almost the whole way around the world before enough tests were available to make this viable. As soon as most places had the tests it was already too late-- they were finding hundreds of cases and there wasn't much point testing. But New Zealand got the virus late enough relative to the tests for this to be useful.

Even then it has very limited use as a tool to keep SARS2 out of your country. All it does is tell you that you don't have SARS2 there yet and so it's worth having a stab at keeping things that way. But the test is only about 70% sensitive. If a plane landed from NY in mid-March, there could easily have been 100 active infections on board, 30 of whom would have tested negative. So if you want to keep SARS2 out indefinitely your only option really is to ban flights from places where the epidemic isn't over and then require everyone to quarantine anyway just in case. This may sounds extremely strict but it is what you need to keep infections at zero.

There is an enormous difference between the strategies you need to keep Covid at zero (which is practically impossible, and most importantly, you have to know when to give up) and just damping it down a bit (which is quite easy and doesn't have to destroy society).

A common skeptic question is that if coronavirus is dangerous and produces severe symptoms, why would they need to test people to see if they have the illness. For instance, the symptoms of illnesses such as bubonic plague and Ebola are so horrific, you wouldn't need to test people to see if they have the illness. Do you feel this is a valid point.

Yes this is a valid point. For some things you want to do a test in order to decide on a treatment. But at the height of the pandemic, we either didn't have tests (in the UK) or the results took about a week to come back. Either is useless if you need to treat someone who is having trouble breathing. Doctors got pretty good at spotting Covid from lung X-rays and were using that for diagnosis.

So what is the point of testing? It can be useful if you are trying to contain an epidemic before it has spread. If you catch cases early, trace contacts, and quarantine anyone who tests positive, even if asymptomatic, it is possible to keep the infection rate in your country very low without having a lockdown. Iceland achieved this. But it's not clear how much more you achieve with this than people just voluntarily quarantining themselves when they have symptoms of any kind of cold or flu, and using simple devices like IR thermometers. Although asymptomatic people and presymptomatic people can spread the virus in theory, it's likely only to be a small amount, so it makes more sense to concentrate your efforts on the symptomatic. This is far easier to do, requires no technology, and has almost no collateral damage to society or health.

The other thing testing lets you do, which is actually useful, is just to know if the pandemic has reached your country yet at all. If not then it might be worth being very careful at your borders and quarantining people coming in. But SARS2 had got almost the whole way around the world before enough tests were available to make this viable. As soon as most places had the tests it was already too late-- they were finding hundreds of cases and there wasn't much point testing. But New Zealand got the virus late enough relative to the tests for this to be useful.

Even then it has very limited use as a tool to keep SARS2 out of your country. All it does is tell you that you don't have SARS2 there yet and so it's worth having a stab at keeping things that way. But the test is only about 70% sensitive. If a plane landed from NY in mid-March, there could easily have been 100 active infections on board, 30 of whom would have tested negative. So if you want to keep SARS2 out indefinitely your only option really is to ban flights from places where the epidemic isn't over and then require everyone to quarantine anyway just in case. This may sounds extremely strict but it is what you need to keep infections at zero.

There is an enormous difference between the strategies you need to keep Covid at zero (which is practically impossible, and most importantly, you have to know when to give up) and just damping it down a bit (which is quite easy and doesn't have to destroy society).

We hear about the number of cases. How are cases determined. Is it based on test results alone or people reporting symptoms without testing being classed as cases.

Traditionally a "case" is someone who's ill and turns up needing treatment of some kind. But in recent weeks the meaning has changed for the purposes of Covid-19 to mean anyone who tests positive.

A clue that most of the positives being found now are false positives is that we keep reading they are "almost all asymptomatic". If they were true positives you would expect the number of asymptomatic infections to be between 30% and 80% as this is the sort of range it was in during the actual epidemic.