Travel after vaccine available but before Vaccine widely available

[lissie]

On 8/7/2020 at 6:38 AM, Wayfairers said:

I thought the negative test might be ok until the issues with the cruise line (cannot remember the cruise line name) that just tried to do a cruise to nowhere and a person who tested negative multiple times preboarding tested positive later on board.  Another time that a pre-test didn’t work......DH ended up with a horrific infection in the mid 1980s when he was tested negative for an infection on Friday for a tonsillectomy on Monday.  But, he had infected tonsils on Monday when they were removed.  Over the weekend he developed an infection but it hadn’t progressed to the point of him or the doctor  knowing there was a problem until it was too late.  He lost an entire semester of college due to that.

Just had a case of COVID in New Zealand The person returned from overseas -went into 14 day  quarantine - tested negative on day 3 and 12 - and then yesterday the child of the family tested positive - now both parents today. Day 3 negative results are not a guarantee of anything. 

[2wheelin]

7 hours ago, lissie said:

Just had a case of COVID in New Zealand The person returned from overseas -went into 14 day  quarantine - tested negative on day 3 and 12 - and then yesterday the child of the family tested positive - now both parents today. Day 3 negative results are not a guarantee of anything. 

Nor 12 apparently? What day did they test positive and had they left quarantine before that? It sounds like at least three people all tested neg twice before all three tested pos? 

[Hlitner]

Nothing more disturbing (to some folks) to put some facts into the discussion.  First, lets talk about testing..  The quick tests (that get results within 15 min - 2 hours) have a false negative rate of up to 20%!  This means that 1 out of 5 folks tested negative could have COVID!  The better PCR tests usually take several days to get results and even they have some false negatives.   NO TEST will test positive for somebody who was recently exposed to COVID.  In fact, even the best PCR tests may not show-up as positive for several days after exposure.  What all this means is that despite any testing program there will inevitably be some cruisers with COVID that will slip through any testing program.  Bottom line for cruising is that folks that get exposed to COVID on their travels to the port will likely not test positive on any test....but they may well develop COVID symtoms while on their cruise.

 

As to vaccines we know from polls that more then half the population (in North America) will not even voluntarily get any vaccine!  To make matters worse, it does seem that if any of the vaccines currently under Phase 3 testing do ultimately get approved they are going to require multiple shots (likely 2 within a month) and will likely have an effective rate that is something less then 100% (the FDA has suggested they would consider approving a vaccine with as little as a 50% effective rate).  

 

If 50% of the population gets a vaccine that is only 50% effective we are still going to have a big COVID problem for many years,  

 

Where does this leave us and the cruise industry?  Some might say in a world of deep doo doo.

 

Hank

[iancal]

It is one thing to have a covid outbreak on land.   It is quite another to have one on a cruise ship.

 

Even if we have had the vaccinations, the very last thing we want is to be on a ship that is barred from many ports because of an on board covid outbreak among those not vaccinated or those where the vaccine was not effective.  

 

Floating around aimlessly on a cruise ship, with or without the vaccination, is not our idea of a good time.

[klkaylor78]

Three things come to mind here

Why are we sure the positive test is the correct result - as stated above all tests have incorrect results both positive and negative. When the disease is rare - as in covid 19 in AUS then it is more likely to have a false positive than a false negative given the rt-PCR test sensitivity ( positive when colonized/infected) and specificity (negative when no virus present). Ill skip the whole discussion of what type of testing we are doing.

 

Many vaccines require multiple dose, don't work for everyone every time, have some rate of unexpected bad reactions, and may have to be repeated years down the road to maintain a protective level of immunity. Some "bugs" cannot be immunized for - HIV, Malaria, common colds, so getting some immunity for some or most of the population still has value.

 

Which brings us to third issue - why have we decided that the goal is protecting everyone from ever being colonized -virus present no illness - or sick, or death from this specific disease. We seem to be down a path that is not realistic with this disease process. We try to prevent diseases such as malaria although it is treatable, and rarely fatal.  It is still present in the amazon but we routinely go there as tourists. We try to prevents, treat it if we get is and essentially ignore the fact that a few people will die of the disease.

 

For covid it is not that much different.  We believe that the virus is highly contagious. It is most likely has a variable rate/severity of illness when infected or colonized, and death rates are higher by age.  It is time to critically review the idea of no infections, no illness, and no deaths.  If we use that level of public health goals we would all have to live in a bubble forever.

 

[ilikeanswers]

8 hours ago, klkaylor78 said:

When the disease is rare - as in covid 19 in AUS then it is more likely to have a false positive than a false negative 

 

11 out of a 1000 false positives is a pretty trustworthy test IMO while still giving you a little hope it could be wrong 😉. 

[lissie]

12 hours ago, 2wheelin said:

Nor 12 apparently? What day did they test positive and had they left quarantine before that? It sounds like at least three people all tested neg twice before all three tested pos? 

One person was in quarantine - the father - he tested negative on day 3 and day 12 - was released from quarantine and reunited with his wife and child. He got tested again as he had symptoms  a few days later, and his close contacts (his family) were tested too which is standard practice - he'd passed it on to his wife and child (who I think are asymptomatic). 

 

He didn't leave quarantine - we have pretty tight security on quarantine hotels 

 

He did  the right thing - seems like they've passed it no one else. They are investigating the source - but the genome sequencing suggests that its the same strain as was found on the flight into NZ - so could just be a very long delay in the disease showing up. Or he could have been infected in quarantine . 

Edited September 21, 2020 by lissie

[klkaylor78]

The actual key data point is positive predictive value - essentially you get the right answer from a diagnostic test

The positive predictive value (PPV) is defined as

PPV=Number of true positivesNumber of true positives+Number of false positives=Number of true positivesNumber of positive calls{\displaystyle {\text{PPV}}={\frac {\text{Number of true positives}}{{\text{Number of true positives}}+{\text{Number of false positives}}}}={\frac {\text{Number of true positives}}{\text{Number of positive calls}}}}

For the case you gave we do not have the specificity but we can use the Covid 19 rt-PCR at 96% for our example and use your give specificty of 99.9

So if there is a 1% prevalence (good guess for Covid) of a disease then 10 out of 1000 will have the disease and and for testing

10 will have the disease, 9 will test positive and one will have a false negative if you test 1000 folks. (sorry cant make half of a person)

with a 99.9% sensitivity (your data point) 11 will be falsely positive and 979 are negative

So we will have 20 positive tests but only 9 will actually have the disease - so over half will be isolated for nothing - not a great test.

So the real issue is that testing asymptomatic patients in a disease with low prevalence the specificity is critical to not have so many false positive that the test is a coin flip (this example) when the test is positive.

People think that testing is perfect but screening testing is much harder to get right than diagnostic testing in light of symptoms.

 

 

 

 

[TheOldBear]

While it is true that antibody production is short lived, cellular immunity provides the 'memory' for the immune system. For examples see https://immunology.sciencemag.org/content/5/49/eabd6160

 

 

There also appears to be some limited evidence that prior infection with SARS-COV-1 or one of the common cold's four known corona virus also offers cellular immunity 'memory'

 

A couple of the vaccine candidates [Oxford, J&J, likely others] were looking for a cellular immunity response, starting from after the phase one trial. [phase 1=does it hurt? ; phase 2=does it do anything? ; phase 3=is it better than doing nothing?]