081 - Measles in Detroit, 1935 I. Factors Influencing the Secondary Attack Rate Among Susceptibles at Risk

Before you can say much about a disease or treatments for it, it's useful to actually understand how it works. One aspect of that is attack rates: When a number of susceptible people are exposed to the disease, how many actually get it?

This was discussed for measles a bit in entry 060, concluding that about 95% of people in cities caught measles by age 15. Here, Franklin H. Top looked at attack rates on a smaller scale, following a number of families in Detroit that experienced one or more cases of measles and seeing how many other susceptible individuals in each family caught it.

Measles is one of the most contagious diseases we know about; this figure compares it to other diseases we know about:

Source: NPR

This is probably why it's showing up so much in the US, despite the very effective preventative measure we have (vaccination).

Franklin had observed high attack rates in Detroit, so much that he says, "parents often consider it unfortunate when all susceptible children in the family do not take the disease at one time, reasoning that it is more convenient to be done with it at once."

So he looked at more than 27,000 cases reported in Detroit in 1935, focusing on families that didn't have any kind of prophylaxis against measles (not that there was anything particularly effective, but we'll talk about that in a later post). He selected a fifth of these families for study, excluding those with no susceptible children or with difficulty in keeping records.

Then he looked at the influence of the sex and age of the primary case of measles, the sex and age of the susceptibles, the period in the seasonal measles cycle in which the case happened, the number of primary cases in a family, and the frequency and intensity of exposures, to see if any of these influenced the attack rate.

Some definitions: the primary case is the first case of measles in a family, so the exposure must've come from somewhere outside. Secondary cases presumably came from the primary case, infecting susceptible contacts in the family. It was assumed (based on previous study) that each case was contagious for 8 days: the exposure period.

There were 1,253 families in the study with one primary case, and 1,380 secondary cases resulting from them. The effect of the sex of the primary case was negligible: the attack rate from male cases was 84.3%, and 84.7% from females. Broken down by age groups, sometimes the male rate was slightly higher, sometimes female.

Looking at age groups of the primary case specifically, cases aged 5-9 years had by far the most susceptible contacts. Those under 1 year or over 15 had too few to make good conclusions. But in the middle, the attack rate was consistently 70-90%. So it didn't seem to matter much; if anything, 5-9 years was the highest, but not statistically significant.

For secondary cases, again sex didn't really matter (84.4% vs. 84.7%). In this case though, age did, a little: contacts between 1-9 years had significantly higher attack rates than those under 1 year, about 85% vs. 45%. I wonder if this is because of maternal antibodies.

Regarding time of year/period within seasonal disease cycle, rates stay constant from January through April at around 86%, then fall off by July until the next winter. Makes sense; measles might transmit better in cold weather, like influenza, or because people are huddled together inside more than when it's warm.

Now about intensity of exposure: comparing families with just one primary case to those with more than one, there didn't seem to be a significant difference in attack rate. It was all maxed out around 80-85%. If a second primary affected anything it would only be another 15%, and those are possibly already somewhat resistant somehow; otherwise they would've caught it from the first primary.

The same is mostly true for those exposed more than once: Not many who have been exposed once will escape, and the attack rate for them when exposed again is much lower. This makes sense; if they escaped the first time, they're more likely somewhat resistant (or records of past illness are incorrect, and they're immune). Still, the attack rate got up to 63% in contacts aged 1-4 exposed twice, so just being exposed once doesn't mean there isn't anything to worry about from later exposures.

Lastly, the influence of intensity of exposure: apparently exposure to multiple cases at the same time doesn't significantly increase the attack rate either.

So overall, it seems like time of year and age of the susceptible person matter the most, while sex and intensity of exposure don't affect much, though repeated exposures can increase the number of victims. And more than 80% of susceptible children exposed to a case of measles will catch it. Pretty hard to prevent.

Reference: Top, F. H. Measles in Detroit, 1935 I. Factors Influencing the Secondary Attack Rate Among Susceptibles at Risk. Am J Public Health Nations Health 28, 935–943 (1938).