Wikipedia:Reference desk/Archives/Science/2014 March 6

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March 6

Soil Quality

What is the maximum value of Sodium Adsorption Ratio (SAR)? — Preceding unsigned comment added by Desalengineer (talk • contribs) 22:15, 6 March 2014 (UTC)

If you look at the second reference (New Mexico State University: Irrigation water classification systems) in the article on Sodium Adsorption Ratio, you'll find Figure 1 on page 3. This shows how to estimate the US Salinity Laboratory Classification on the basis of SAR and Electrical Conductivity (EC x 10^6). The left axis of this chart is SAR, and you'll see that it goes up to a maximum of about 30. This suggests that 30 is the maximum that you're likely to encounter in most circumstances. Note, though, that because of the way the ratio is calculated, there is no theoretical maximum value. From www.salinitymanagement.org I notice that "As a rule, water that has an SAR below 3 is safe for irrigating turf and other ornamental landscape plants. Water that has an SAR greater than 9, on the other hand, can cause severe permeability problems when applied to fine-textured soils (for example, a silty clay loam) and should be avoided." I also randomly looked at some USGS reports, and on the basis of this very small sample it looks likely that values above 3 are comparatively infrequent. RomanSpa (talk) 22:35, 6 March 2014 (UTC)

Vaccinations

If a person is vaccinated from a certain virus, can they still transmit that virus even if they don't become I'll from it themselves as a result of the vaccination? I.e. can someone not vaccinated from the virus become ill as a result of close contact with te vaccinated person, who also came into contact with the virus. 82.40.46.182 (talk) 22:35, 6 March 2014 (UTC)

As you will see from our article on vaccination, there are four types of vaccine currently in use. These are: inactivated vaccines, attenuated vaccines, virus-like particle vaccines and subunit vaccines. Since the virus in an inactivated vaccine has been killed, there should generally (so long as the virus has been correctly inactivated) be no risk of infection or transmission. Since virus-like particle vaccines and subunit vaccines don't use the viruses themselves, there is no risk of infection or transmission for those kinds of vaccines either. This only leaves attenuated vaccines. These work by breeding a very mild strain of the virus in a sterile environment; when this is delivered as a vaccine it stimulates the immune system in the usual way, giving protection against not just the mild strain, but also against nastier strains. When you are vaccinated with an attenuated vaccine you have a mild form of the virus inside you, and this can potentially infect other people, but this is a good thing, because it means that they also develop immunity against the mild strain, and thus also against its nastier siblings. The resistance you build to the mild form protects you against the nasty form. There is a tiny possibility that the virus in an attenuated vaccine might change into something more virulent, due to the normal mutation process that is going on with viruses all the time, and under those circumstances you might infect someone else, but this sequence of events is very unlikely - from the references in the article, we can see that in the case of a polio vaccination campaign there was a partial reversion to virulence that affected only four patients in a country of nearly 100 million people, and even then the problems that arose were significantly less than the original disease. Note that for vaccination to work you have to have a functioning immune system, of course, which is why immunocompromised people should talk to the doctor before being vaccinated. If you are going to be vaccinated with an attenuated vaccine, and are regularly in contact with immunocompromised people, the best thing to do is to talk to your doctor, and ask about the particular vaccine you are considering. In general, though, the answer to your original question is: there is, in some cases, a tiny possibility that someone might become ill as a result of close contact with someone who has been vaccinated with an attenuated vaccine, but the probability is so small that except in the special cases just mentioned it's not worth losing any sleep over. As always, note that Wikipedia is not a medical professional. If you have particular medical concerns, your doctor will be able to give you appropriate guidance. RomanSpa (talk) 23:58, 6 March 2014 (UTC)

Sure, it's possible, but very unlikely, because most human-infectious viruses can't survive very long outside the human body. For influenza, it's usually a matter of hours, unless frozen or specifically preserved. To elaborate on RomanSpa's good answer and refs: suppose I got a flu shot, and you have this year's flu. You sneeze in my face while you are infected, and then I leave that room and immediately start kissing RomanSpa. I would be protected from the infection taking hold in me via my vaccine, but it is conceivable that I could transmit the virus to RomanSpa, because my vaccine won't due anything to combat a lingering contamination on my face. In this (obviously contorted) example, I could act as a vector_(disease) to infect RS, even though I am vaccinated, and you have had no contact with RS. But, all that doesn't really matter in the aggregate. If enough people are vaccinated, herd immunity kicks in. SemanticMantis (talk) 00:28, 7 March 2014 (UTC)

For clarity, I should just say that SemanticMantis would have to take me out for a nice meal first, or at least get me very drunk. RomanSpa (talk) 01:37, 7 March 2014 (UTC)

I'd say yes. First, the vaccine can simply be ineffective or only partially effective - for example, I've heard figures around 70% for the antique influenza vaccines we're still using. Somewhere between effective and ineffective there ought to be some people who get just some symptoms and shed just some virus.

Another option is that (as suggested above) the attenuated vaccine can get out of control -- this is the case with the polio vaccine, which periodically would start mini outbreaks in the vaccinated populations. Once, public health officials had imagined that they would wipe the virus out "any year now", and had written volumes on how to decide exactly when to switch over from the more effective live vaccine to the non-risky killed vaccine. Then the U.S. sent a team to do polio vaccination while collecting information about Osama bin Laden, the Pakistanis stopped the program, the remaining virus in those hostile regions spread to Israel and various other places around the Mideast... now they're going back and revaccinating people with the live vaccine. Wnt (talk) 00:35, 7 March 2014 (UTC)

Yes this can happen and I know a victim of it. The polio vaccine is live and can be spread to carers who change the nappies of children who have recently been vaccinated: parents in the UK are now advised to wash hands well after changing dirty nappies to prevent this. As I said, I do know someone who caught polio from changing his granddaughter's nappy. --TammyMoet (talk) 10:35, 7 March 2014 (UTC)