Respiratory syncytial virus (RSV): here today, gone tomorrow

I’ve written before (here, here, and here) about RSV, one of the most common causes of respiratory illness in infants and toddlers, and the most common cause of illness severe enough to land them in the hospital. It’s so common that virtually 100% of children have gotten the infection by the time they’re two years old. RSV generally causes an illness called bronchiolitis. In this post I’ll tell you about why it causes such sudden and explosive epidemics.

I’ve hardly seen any RSV yet this year. But all of us know it will come; generally we see a few cases, quickly followed by an explosion of cases. The way RSV behaves in the population is fascinating. It’s also utterly predictable, based upon what we know about the properties of the virus and our immune response to it.

The first thing to know is that RSV is highly contagious — one of the most contagious of all viruses. It’s spread by droplets of respiratory secretions, and it can survive for several hours at least on objects, such as shared toys or cookies. Its attack rate, the number of people who are susceptible to the infection and who get it if exposed — is well over 90%. So once cases appear, if there is a large population of people susceptible to it, we would predict a lot of infections.

The second thing to know is that there is always a large number of susceptible people. This is because our immunity to RSV is not good; most of us, especially if we are exposed to small children, get the infection every few years. For some reason RSV doesn’t induce a very good immune response, so when we get it we don’t develop very good protective antibodies to it. This is why we haven’t been able to develop a vaccine against it.

It also explains why infants get it so easily. Babies are born with a dose of antibodies they get from their mothers, protection that lasts a couple of months or so. In the case of RSV, though, mothers can’t give them this protection. So they’re all susceptible, and it’s generally the infants, especially those born early, who have the most trouble from it. (Adults generally get only mild to moderate cold symptoms.)

So why do we have the explosive epidemics from RSV? The answer is that each year a whole new crop of susceptible infants are born for the virus to infect. That, plus the high attack rate, causes RSV to rampage through the population once a few cases appear.

Although all children will eventually get RSV, there are a few things you can do to reduce the chances of your infant getting it during the typical epidemic of mid to late winter and early spring. Simply postponing infection until your child gets out of infancy is very helpful, because older children rarely need to come into the hospital for treatment. Avoid close exposure of your infant to anybody who has cold symptoms, and have everybody wash their hands before handling your baby.

In sum, although RSV infection is a rite of passage in childhood, there are a few practical things you can do to keep your child out of the hospital.