I've been looking at the effectiveness of Flu vaccines, which are in my subjective opinion abysmal, and the concept of herd immunity. I see many articles which argue that we all need flu shots to help support the weaker individuals via herd immunity. However, I see very few documents actually indicating how effective this process is.

Most of what I have found on the topic is interested in the Herd Immunity Threshold (HIT), which appears to be the point where vaccinations drive the Rq of the disease (reproductive rate in the presence of the vaccinations) below 1 such that the disease spreads to fewer and fewer people with each generation, like removing the heat from a fire such that it cannot sustain itself.

There are two particular aspects of this which I believe are required to answer the question which I cannot seem to find a good answer for, which have been preventing me from answering my question:

  • What is R₀, the basic reproductive rate, for influenza? I have seen estimates ranging from 1.2 to 3. As best as I can tell, even a poor vaccine would be very effective if it was 1.2 but it seems like it would do very little against the higher R₀ values.
  • How substantial of a knee in the curve is there regarding the number of immunized individuals vs. the herd immunity effects? If there is no substantial knee in the curve, that should indicate that there are valid herd immunity arguments for influenza even if the vaccines are substantially ineffective. If the knee in the curve is sharp, then herd immunity arguments would only be valid if we could reach that threshold.

It appears some of these questions may have been answered in "The vaccination coverage required to establish herd immunity against influenza viruses" by PedroPlans-Rubió in 2012, but I do not have any access to Preventive Medicine, the journal it was published in.

  • $\begingroup$ The flu vaccine is a poor choice for this kind of analysis because of the tremendous number of variables involved: what was the dose inhaled/how intact is the 'victim's' immune system/how close is the match between the vaccine and the circulating (ever mutating) virus, etc. The world's epidemiologists struggle with this anew every year. The question is better for a virus like varicella. $\endgroup$ Jan 16, 2018 at 19:06
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    $\begingroup$ @anongoodnurse True, it might be easier to analyze another virus, but influenza is the one I am interested in. It is actually the variability and uncertainty surrounding influenza that draws my attention to it. I see a great deal of certainty expressed regarding the influenza vaccine, but I'm having trouble finding numbers that back those certainties up. For example, the anti-vaxers love to tout the ineffectiveness of the vaccine (perhaps 10% this year!), but I find it hard to find out what effectiveness is actually needed. Maybe 10% is actually enough if we all do it! $\endgroup$
    – Cort Ammon
    Jan 16, 2018 at 19:11
  • $\begingroup$ I can't answer; the effectiveness changes every year. I can say, though, that as a young doctor, I recommended the vaccine but didn't get it. Four years in, I contracted influenza. I never missed a flu shot again, and never got the flu again (even though I treated such patients every year.) Decades passed. I contracted influenza again this year - only the second time in my life - in a year that the vaccine is a notoriously poor match. Maybe because I received the high-dose vaccine, my course was very mild. Others who have been immunized (chiefly infants) have died. How can you get hard facts? $\endgroup$ Jan 16, 2018 at 19:22
  • $\begingroup$ Cort, you might want to look at the Cochrane Report. Keep in mind, though, that they don't only include influenza A&B (what the vaccine covers.) $\endgroup$ Jan 18, 2018 at 2:00

1 Answer 1


Quoting Jhung et al. 2011 talking about H1N1 2009 epidemics

Although estimates of R0 from past pandemics vary, the R0 of previous pandemic influenza viruses generally range from 1.5 to 1.8 for the 1957 H2N2 and 1968 H3N2 pandemic viruses, and 1.8–2.4 for 1918 H1N1 influenza A strain, with a high estimate of 5.4 by Andreasen et al [see also White et al]. During the current pandemic, White et al used CDC case-based data from 1368 confirmed and probable cases with a date of report on or before 8 May 2009 and estimated the reproductive number of pH1N1 virus to be between 2.2 and 2.3. Estimates decreased to 1.7–1.8 after adjustment for increased case ascertainment during the initial pandemic period. In a sensitivity analysis making use of previous estimates of the mean serial interval, White et al estimated that the reproductive number was between 1.5 and 3.1. Fraser et al used data from the initial outbreak in Mexico to estimate R0 in the range of 1.2–1.6. Yang et al used reported case clusters in the United States to estimate R0 to be 1.3–1.7. Most estimates of R0 for pH1N1 virus, therefore, have indicated that the virus was at the low end of transmissibility, compared with the strains that caused the 1918 pandemic, and was comparable to or slightly less transmissible than the strains that caused the 1957 and 1968 pandemics.


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