How Contagious Is That Dinner Party? And How Best To Evade Friends' Bugs?

"This is not a question for an expert on etiquette!" I expostulated. "It's a question for an Infectious Disease specialist!"

The trigger for that objection: A query to Social Qs, the New York Times etiquette column on "awkward situations." The writer describes a group of 10 close friends who meet regularly for dinner; one, who is immuno-suppressed, asks that another member who is getting over the flu and still on antibiotics keep 12 feet away and avoid touching anything she may eat. The reader asks: "Shouldn't one of them have declined the invitation? But which?"

The columnist responds that no, no one needed to bow out, and that this distance-setting arrangement seems a good compromise, adding, "I hope the person with the flu called her doctor to make sure she was no longer contagious — for everyone’s sake."

Surely you can understand my frustration. Fine, the flu patient could call her doctor, but what about the rest of us, hungry for more general knowledge on contagion for our own social gatherings? Why not answer the obvious questions? Like: Is 12 feet really far enough to avoid flu germs? Are you still contagious when you're finishing a course of antibiotics?

Oddly, the day after I read that column, a similar situation played out at my house: One friend was getting over a respiratory infection, still coughing, and another friend regretfully said she could not stay and chat at the dining table, for fear of carrying a germ to an immuno-compromised loved one.

That did it. I called CommonHealth's go-to guy on infectious disease questions of public interest, Dr. Ben Kruskal, chief of infectious diseases at Harvard Vanguard Medical Associates, and shared my annoyance. Actually, he gently corrected me, this is an issue of both medical science and etiquette. Our conversation, lightly edited:

Dr. Kruskal: You need to have the facts and then you can figure out the etiquette in light of them.

First, when it come to infectious disease transmission, we know a fair amount, but there’s a lot that is still argued over. So let’s take flu as a good example. We know that there are multiple mechanisms by which flu is transmitted, or by which you could postulate reasonably that it might be transmitted:

• Physical contact: You've got germs on your hands, you touch somebody else's hands or face. Or indirect contact — you touch your face, and then touch the doorknob. A few minutes later, someone else touches the doorknob and then their face.

And there are two different mechanisms of airborne transmission:

• Respiratory droplets, which fall to the ground pretty quickly after they leave your mouth and nose. People argue about the distance they can travel — some people say three feet, some say six feet. Six is a very conservative estimate.

• And then there's what's called true airborne transmission (technically, droplet nuclei), which is the mechanism by which TB, measles and chicken pox are all transmitted. And that's the kind that can go much longer distances and can linger in the air for a long time afterward.

So which is flu?

Flu looks like it’s probably mostly droplets, with some contact, and then there’s a lot of debate in the medical literature about whether there’s some component of airborne transmission or not. If it's there, it's probably not huge — we're arguing whether it's .1 percent or 1 percent or 5 percent, but it’s probably not more than that.

What else should we know?

The relative amounts of transmission by droplets or contact vary among different germs. We know RSV, another common respiratory virus, is transmitted much more by contact but also by droplets.

So in general, can we say most viruses are mostly transmitted by droplets?

I would say that among common garden variety infections, not just respiratory illnesses but a lot of intestinal infections — the vast majority are transmitted either by physical contact or by droplets.

It's very likely among a group of 10 people that a couple of people might be shedding some virus.

So to the social situation: These are very practical questions that I get asked by patients all the time. I also get every variation on this you can imagine posed to me by colleagues as well. Can grandparents visit a kid who just had the chicken pox vaccine, which is a live vaccine? A teenager has been exposed to pertussis but hasn't got any symptoms — can the immuno-compromised aunt come visit? There’s a million variations on this theme and it almost always involves social considerations.

So if I were the physician adviser to this group of 10 people, I'd say it’s not a slam dunk by any means, how to decide this.

If I were really to counsel them in the most accurate way, I would need to know something about the nature of one person’s immuno-compromise and I'd need to know as much as is known about the respiratory condition that the other person has. But at this time of year we also have to throw something else into the mix: It's very likely among a group of 10 people that a couple of people might be shedding some virus or other at a given time.

So even someone who’s not aware they’re sick or has something so trivial they haven't really paid attention has a very decent chance of having a virus. And remember, with a lot of illnesses you're infectious before you're aware of symptoms.

I must say, it's toward the end of the flu season but this is the moment when it feels like everyone I know has been getting sick...

Right. Though that's probably a little bit of a statistical blip. There's still a lot of flu around but it is definitely on the wane...

So you should assume someone is shedding something - but you can't just hide yourself away until June...

There are a lot of different ways of looking at this particular social situation. You could say, you shouldn't forgo this gathering if you're going to be out in any social setting where you're going to be within three feet face to face of people or certainly touching people, shaking hands. You could argue that yes, it's a gathering of close friends, they may be hugging, kissing, whatever, but then you just say, 'If you’re sick, please stay away.' So you could take very reasonable precautions under those circumstances.

Being guided by the knowledge that what you're trying to avoid is being within three feet or touching somebody?

Exactly, and of course, the corollary — you don't share food or drink, which is an indirect form of touch.

That's helpful. I was groping for where to set the boundaries and it seems clear: Don't go within three feet or touch.

Right. This is actually a much more controlled setting than going to the mall. If you go to the mall you’ll be within three feet of other people all the time, and you have no knowledge about their illness status.

And again, if I were the doctor advising the immuno-compromised person, I’d need to know the nature of that immuno-compromise. There are some people to whom I’d say, 'Don’t go out.' And other people to whom I’d say, 'It doesn't matter, go anyplace.'

What if you're just speaking to the worried well, who just really don't want to want to get sick again this year?

The best parallel to this is when I talk to parents of newborns. What I say to them is, 'It's okay to go out in public as long as you're prepared not to let any strangers come within three feet of the baby. And sometimes, of course, that means you have to be rude.' But the flip side is that it’s perfectly reasonable to go out under those circumstances. And then, when it comes to physical contact with people you know, visiting relatives even in your own home, the rules are very simple: Nobody who’s sick gets within three feet, and everybody who’s going to touch the baby, even if they're well, has to clean their hands. If you do that it’s not 100 percent — there's no way to get 100 percent protection other than being a hermit — but if you do that, you reduce your risk very, very, very considerably.

And I've wondered for a long time: This friend who said, 'I've been on antibiotics for three days so I'm not contagious anymore' — is that actually realistic?

The short answer is, probably the friend had a viral infection, in which case antibiotics don't mean anything.

Again, there are lots of nuances. The short answer is, probably the friend had a viral infection, in which case antibiotics don't mean anything. The vast majority of respiratory illnesses with which people are actually walking around are viral.

So the antibiotics don't matter then. So then the question is, how long do people usually shed virus?

And again, it varies tremendously and our knowledge of it is relatively limited as well.

But it certainly would be for days after symptoms show up?

When we're looking for a rule of thumb, we usually say that it's as long as there are significant symptoms — and fever is the one that’s most objective. So when the fever is gone, people tend to be shedding much much much less virus.

Another nuance here, so you understand the big picture: When we talk about shedding virus, we can measure that in various ways, and most of the time we look at the presence of some piece of the virus that we can actually measure, but we're not actually looking at transmissibility. So it's much easier for me to take a swab from your hand or your nose and put it in a test tube and say, 'Oh, there's flu RNA there that I can detect and therefore you’re shedding virus on your hands or nose.' But in fact, that doesn’t always correlate very well with transmissibility. There are several issues including the question of how much it takes to transmit a given bug. For some germs, we know one bug is enough; others it takes a thousand or ten thousand before the average healthy person gets sick. In addition, most of these techniques are chemical in some way, which don't necessarily detect an organism that's healthy enough to be transmitted. It may detect a dead germ or even a fragment of a dead germ.

I'm surprised that since transmission is so incredibly important, there is — it sounds like — something of a research gap there. Or is it something that we can't study more?

We can study it more and as I said, we know a fair amount but there are gaps. This question of how much flu transmission is airborne is one of them. We know flu is mostly respiratory droplets. We know a substantial minority is contact. And how much is airborne — that's a question. Again, we know it’s not a lot but not exactly how much it is. And of course, those questions are hard to answer for all kinds of reasons, including, among other things, that we're talking about multiple different strains of flu at a given time, and different strains of flu year to year. So you can often answer a very narrow question very well, but the larger question, which is the one you really care about, is very hard to answer.

Well, that's an answer in and of itself: That there's not going to be a simple, general answer to this — you need the specifics...

Readers, helpful despite the limitations? Lingering burning questions?