HSCI 408 Polio and herd immunity

Herd immunity (plain definition)

A microbe stops spreading when enough people in a community are immune.

Topley & Wilson (1923) on herd immunity

Spread of infection occurs with difficulty among a population that has been actively immunized.

Last's Dictionary of Epidemiology: herd immunity

A group's resistance to invasion and spread of an infectious agent, based on the resistance of a high proportion of its individual members.

Wan Quan (1549, Douzhen Xinfa)

Noted that once someone has suffered smallpox, they will not be infected with it again (early idea of immunity).

William Farr (1840)

Said smallpox would be disturbed, and sometimes arrested, by vaccination that protected part of the population (early herd immunity idea).

Epidemiologic triangle: three parts

Agent, host, environment.

Who described the epidemiologic triangle?

Wade Hampton Frost (Am J Epidemiol 1976).

Triangle: agent

A microbe capable of causing infection and disease.

Triangle: host

A host population with sufficient numbers of susceptible people.

Triangle: environment

Conditions in the environment that bring the agent into contact with infectible hosts.

Hedrich (1933) measles in Baltimore

Measles epidemics rose and fell with the supply of susceptible (intact) children under 15; cases spiked then susceptibles dropped, then rebuilt.

Fox et al. (1971)

Simple thresholds do not adequately capture herd immunity in diverse populations.

Fine (1993): measles herd immunity estimates

55% (Baltimore observations), 70-76% (population models), 85% (West African simulation, annual immunization of susceptibles), 94-96% (population models). Different populations and models gave different estimates.

Most contagious disease on the NYT 2020 chart

Measles (about 15 people infected per case).

Pasteur's 1881 anthrax trial

Pouilly-le-Fort, France. Vaccinated animals survived; unvaccinated sheep and goats died.

Toussaint (1880)

Protected animals against anthrax with heat-treated material, before Pasteur's famous 1881 demonstration.

Galtier (1881)

Immunized sheep with intravenous injection of rabid saliva; an important precursor to Pasteur's rabies vaccine.

Duclaux and the chicken cholera story

Popularized the account of aged chicken-cholera cultures leading to attenuation (an accidental discovery story later questioned by historians).

Main message of 'The Making of Modern Vaccines' slide

Vaccines were built incrementally through many hands; Pasteur's breakthroughs grew from decades of shared work.

Jakob Heine (1840)

Recognized poliomyelitis as a distinct condition.

Karl Landsteiner (1908)

Showed polio is caused by a virus: filtered out larger microbes, injected the fluid into two rhesus monkeys, both developed paralytic polio.

March of Dimes

Campaign by the National Foundation for Infantile Paralysis (founded by FDR; Basil O'Connor led it) to mobilize a nation against polio.

Park-Brodie vaccine (1935)

Inactivated virus (formaldehyde) from infected monkey spinal cords; made before all three types were established; about 3,000 children; inconclusive and abandoned.

Kolmer vaccine (1935)

Live attenuated polio vaccine; paralysis in about 1 per 1,000 recipients, some fatal; abandoned after intense criticism.

Salk vaccine (1955)

Inactivated virus (formaldehyde), grown in monkey kidney cell cultures, all three poliovirus types, about 1.8 million children in field trials, licensed in 1955.

Salk vaccine efficacy reported in 1955

80-90% (NYT, April 13, 1955).

Enders, Weller, Robbins

Won the Nobel Prize (work growing poliovirus in cell culture that made vaccine development feasible).

Isabel Morgan

Linked on the slides to the three polio serotypes.

George Gey

Associated with HeLa cells.

Cutter Incident (1955)

Problem with Cutter Laboratories' Salk vaccine (live virus not fully inactivated) that caused polio cases.

Martinez-Bakker et al. (2015): cause of 1950s polio epidemics

Rising births (baby boom) increased the number of susceptible children; many more were infected than reported.

Oshinsky quote (Polio: An American Story)

Black children were widely believed to be less susceptible than White children, and therefore less in need of care.

IPV (killed polio vaccine)

Contains three serotypes of vaccine virus, grown on monkey kidney cells, inactivated with formaldehyde.

OPV (live polio vaccine) downsides

Shed in stool for up to 6 weeks; causes paralysis in about 1 in 2 million vaccinated children; can revert to vaccine-derived poliovirus.

Where is OPV used?

Still used in low- and middle-income countries (highly effective immunity); discontinued in the US and Canada.

Wild polio cases in 2023

12 total (6 Afghanistan, 6 Pakistan), all type WPV1 (all cases since 2013).

Which wild polio serotypes are eradicated?

Types 2 and 3 (only type 1 remains).

Alfred Evans (1985)

Listed factors favoring eradication of a disease (AJE: 'The eradication of communicable diseases: myth or reality?').

Evans: factors favoring eradication

Human-only host, clear clinical disease, few subclinical cases, no long-term carriers, one serotype, short infectious period, long-lasting immunity, seasonality, stable vaccine that mimics natural infection, cost effective.

Why is polio harder to eradicate than smallpox?

Three serotypes, many subclinical (silent) infections, and fecal shedding / vaccine-derived virus.

US disease declines (Roush, JAMA 2007)

Smallpox eradicated; diphtheria 100%; polio 100%; rubella 99.9%; measles 99%; mumps 95.9%; pertussis 92.9%.

US measles resurgence 1989-1991: peak

27,808 cases in 1990.

Measles resurgence: what did NOT explain it

No change in virulence, stable vaccination rates in 2-year-olds, imported cases did not increase, and it preceded the fraudulent MMR-autism article.

Measles resurgence: likely cause

Baby boomlet (1984-1992): rising births increased susceptible density (Lanphear et al. 1997 model).

Canada measles timeline

Vaccine 1963; routine 1-dose MMR 1983; routine 2-dose MMR 1996/97.

US measles timeline

Vaccine licensed 1963; second dose recommended 1989.