Another reason why mandating flu shots is stupid

I wanted to share with readers a simple, back of the envelope calculation that points out the folly of trying to control respiratory illness in healthcare workers by mandating influenza vaccine.

Here are the assumptions for the calculation:

• Our hypothetical hospital has 5,000 healthcare workers
• It’s a nonpandemic influenza season, in which 7% of the population (including healthcare workers) gets an influenza-like illness (ILI), of which 7% is due to influenza (these estimates are from the control arms of 95 influenza vaccine trials involving 1 million subjects over the course of four decades). Now of course the 7% of the 7% (those with influenza) can be reduced by influenza vaccination.
• The hospital has a baseline influenza vaccination rate of 70% without mandating the vaccine
• We’ll generously assume that influenza vaccine is 90% efficacious at preventing influenza (though it has no impact on non-influenza ILI)
• Presenteeism is 70% (i.e., 70% of healthcare workers come to work when they are sick)

Now let’s take 2 different approaches to reduce the risk of transmission of ILI to patients. In the first approach, we’ll mandate influenza vaccine and achieve a vaccination rate of 98% (we'll assume that 2% of HCWs have a contraindication). In the second approach we won’t mandate vaccination and we’ll maintain the vaccination rate at the baseline of 70%, but we undertake an educational campaign to reduce presenteeism.

So, the question is this: what reduction in presenteeism would have the same impact as achieving 98% influenza vaccination in terms of the number of HCWs at work with ILI? The answer is an astonishing 1% absolute difference. Reducing presenteeism by 1 percentage point (from 70% to 69%) would have the same impact as increasing vaccination from 70% to 98%. So in our 5,000 employee workforce if we could get 4 HCWs with ILI to stay home it would have the equivalent effect of mandating influenza vaccine. 

Now one could argue with the assumptions and we could re-run the numbers using different percentages for any of the variables. But the primary and irrefutable message is this: pathogen-specific interventions (i.e., vertical approaches) for controlling transmission of infection in the hospital are inefficient when compared to multipotent, horizontal approaches. And remember that a sizable fraction of the patients are vaccinated against influenza (we don't have vaccines for the rest of the ILIs), which actually increases the differential impact of the strategies in favor of reducing presenteeism.

Unfortunately, the horizontal approach to controlling ILI won’t make any companies any money, and in our sound bite culture, the path of least resistance (which SHEA and others have fallen prey to), is to simply call for mandating flu vaccine.

Let me once again emphasize I am not anti-vaccine. I get a flu shot every year. But I think the costs of forcing resistant healthcare workers to get vaccinated in terms of unintended consequences outweigh the benefit. So I can live with 70% compliance and find other ways to protect our patients.

P.S. my calculations are below:

	Baseline	Alternative strategies
	70% vaccinated, 70% presenteeism	98% vaccinated, 70% presenteeism	70% vaccinated, 69% presenteeism
Total HCWs	5,000	5,000	5,000
Vaccinated HCWs	3,500	4,900	3,500
Unvaccinated HCWs	1,500	100	1,500
HCWs with non-flu ILI (6.5%)	325	325	325
Unvaccinated HCWs with flu (0.5%)	7.5	0.5	7.5
Vaccinated workers with flu (0.5% x 10%)	1.75	2.45	1.75
Total HCWs with ILI	334	328	334
HCWs at work with ILI	234	230	230