Herd immunity, epidemics, and vaccinations: it boils down to math

A recent article brought to my attention (thanks to Paul Susman) presented an important conceptual and mathematical framework to assess the potential communicability of microorganisms (see https://portside.org/print/2017-11-06/unforgiving-math-stops-epidemics​ ). And I think this article provides a good basis for discussing immunization with patients, and in particular the importance of flu vaccine, given the large number of people who decline it (the groups who "I never get the flu" or "I got the flu from the vaccine" etc)

Details:

--there are some specifics of the microorganism infectivity that govern its spread:

    --the R₀ of the microorganism (basic reproduction number) varies widely, and is not necessarily what we in clinical medicine would have assumed. And knowing this number allows easy calculation of the level of herd immunity needed in the population to prevent its spread (herd immunity being that % of the population immune to infection, typically because of prior infection or immunization, such that a new infection cannot spread in the community):

        --Measles (perhaps the most contagious organism): R₀=12-18, so need about 92-95% of the population to have effective immunity to prevent its spreading

        --Ebola:  R₀=2, so need about 50% of the population to have effective immunity (this number may seem lower than expected; but Ebola is transmissible only when patients are very sick (eg, spreads to health workers, burial workers), and effective quarantine therefore helps (eg initial calculated projected deaths attributable to Ebola were on the order of 1.4 million, though there were <30,000 actual cases)

        --Flu (from the 1918 flu pandemic): R₀=2, so need about 50% of the population to have effective immunity

Commentary:

--the above explains an issue of childhood diseases: there often is a lag between epidemics. This lag depends on the % of the herd immunity needed to effectively block the disease from spreading (calculatable from the R₀). So, perhaps it takes 5 years for some of these diseases to allow for enough nonimmune newborns to grow and dilute the herd immunity, either because there is no vaccine available, or because the combination of native cases and vaccination rates together descend below that herd immunity threshold.

--there are many ways to mitigate these epidemics, which depends on the characteristics of the population and of the microbe:

    --vaccination: unfortunately, the levels of vaccination are often much lower than we need to sustain herd immunity. For example, there was a measles outbreak in Disneyland in California, but studies have suggested only about a 50% vaccination rate.

    --understanding the transmissible mechanisms of the microbe: as above, knowing that Ebola is transmissible late in the disease allows for measures to reduce the transmissibility. HIV, on the other hand, needs different methods (barrier protection, drugs) to prevent transmission, since it is quite transmissible throughout its infection, and especially so very early after infection. Or the recent cholera epidemic in Yemen, where the mathematical model was pretty accurate in predicting the number of individuals ultimately infected, probably related to the lack of implementing effective means to control the epidemic (the Ebola one, in contrast, elicited huge concern/fear by the Western countries, leading to lots of aid, shipments of protective gear, etc)

--personal protection: though one concern to relying on personal behavior change is its difficulty to implement reliably: eg, hand-washing has been documented as being extremely effective in reducing infectious disease spread, yet health care professionals (a very high risk as well as knowledgeable group) still have <50% adherence (which is lower than most vaccination rates)

--one lingering concern I have (not sure this is true): if we have an effective vaccine with good uptake by the population, will the individual (and therefore herd) immunity last as long as it did historically with natural disease??  I was concerned about this with the institution of the varicella vaccine because: 

    --varicella has much more morbidity/mortality in adults than in kids (ie, delaying its development is worse…)

    --the situation at the time was that varicella virus was quite prevalent in the community, with adult immunity rates in the 90% range

    --this presence of natural virus was continually stimulating and boosting the immune system response to the virus (ie, would getting the virus really lead to life-long immunity if there were no repeated exposure/boosting of the immune system?)

    --and giving the vaccine would deplete the native virus in the community and its boosting effects

    --so, would giving everyone the vaccine lead to dramatic decreases in the virus in the community? And would this lead to waning future immunity because there was no boosting of this immunity by the virus? And would this lead to more varicella outbreaks perhaps 20 years later (which has much more morbidity/mortality) for perhaps either those who had been vaccinated or those who had natural varicella disease or both, perhaps by exposure to someone with active varicella infection coming here from a different country where the virus was still prevalent?

    ​--a case-in-point is the pertussis vaccine, which gives only limited protection (in adults, seems to be in the 3-5 year range), after which outbreaks can occur and infect these previously-vaccinated individuals. Though the data are somewhat conflicting, one study looking at old data suggested that there was on average at least a 30 year period of natural immunity following a natural infection (see http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1000647 ), though there are also concerns about the immunogenicity of the acellular vaccine we now use.

--and, of perhaps the most importance regarding flu vaccine: the above reinforces the issue of vaccination being a social issue/imperative vs an individual one. we clinicians should probably emphasize that vaccination of an individual really protects a much broader group of people overall, and specifically those at highest risk of a bad outcome from the disease (especially the old, who do not mount as much of a response to the vaccine and studies have shown that immunizing their caretakers is much more effective than the patient, though this was before the higher potency vaccines currently being used; the young kids; and those with chronic medical conditions).  One technique is to say "I got the flu vaccine, in part to protect me from this potentially very serious/disabling disease, but also to protect me from giving the flu to older patients who could get it from me, who are much more likely to die from the virus, and who do not develop as protective an effect from the vaccine"​

If you would like to receive regular emails with these blogs, please let me know at gmodest@uphams.org