yellow map
madeleineshort

Will states with low vaccination coverage affect herd immunity in states with high coverage?

Question: How will states with low vaccination coverage (like Mississippi) affect herd immunity in states with high vaccination coverage (like Maryland)?

Answer: Herd immunity is the idea that once enough people have immunity to a virus– through vaccination or infection– there will be so few “hosts” for the virus that community transmission will be curbed, chains of transmission will end, and even those who have no immunity will be protected.  I know we hear a lot about the 70% threshold for herd immunity, but it’s important to remember that achieving herd immunity is dependent on four factors: 

  1. How contagious a virus is. With the Delta variant far more contagious than even the Alpha variant (which itself was 50% more contagious than the original strains), the virus can swiftly move from one person to many.
  2. How much immunity is conferred by the vaccine (or previous illness). FDA approved vaccines– Pfizer, Moderna, and J&J– have high effectiveness.  Even against the Delta variant, vaccines show high, albeit reduced effectiveness.  Risk of breakthrough infection and reinfection exists, but remains low based on what we currently know.  
  3. The social networks within the population (e.g. how people mingle). The more complex a social network, the more opportunities for the virus to find a susceptible host.  
  4. The distribution of the vaccine/immunity within the population (e.g. heterogeneity of the population vaccinated/infected). If immunity is clustered in only certain populations, herd immunity cannot be achieved for the broader population.

Turning directly to your question, low levels of vaccine derived immunity in Mississippi will directly affect communities and individuals in Mississippi who remain susceptible to the virus; we’re talking high risk of infection and onward transmission.  As challenging as this situation is for susceptible individuals in Mississippi, it also has ripple effects that could impact communities in Maryland, including through widespread mingling across porous state borders.  One of the major challenges– for Mississippi and the world– is that the more opportunity SARS-CoV-2 has to spread, the more opportunity it has to mutate into potentially more dangerous variants and/or variants that current vaccines cannot protect against.  For these reasons, it’s imperative that those of us who can get vaccinated do get vaccinated!