Tag Archives: varicella virus

John Enders: “The Father of Modern Vaccines”

John Enders (1897- 1985) was one of the subjects of a recent posting, Vaccine Research Using Children (1). In the 1950s, Enders used severely handicapped children at the Walter E. Fernald State School in Massachusetts to test his measles vaccine—a vaccine that may have saved well over 100 million lives. Irrespective of the ethical issues raised by the incident at the Fernald School, Nobel laureate John Enders was one of the most highly renowned of virologists, and there is much more to his story, some of which is told here.

John F. Enders, November 17, 1961
John F. Enders, November 17, 1961

Enders grew up in West Hartford, Connecticut. His father, who was CEO of the Hartford National Bank, left the Enders family a fortune of $19 million when he passed away. Thus, John Enders became financially independent, which may help to account for his rather atypical path to a career in biomedical research.

Enders was under no pressure to decide on a vocation, and had no particular objective in mind when he enrolled at Yale University in 1915. In 1917 (during the First World War) he interrupted his Yale studies to enlist in the Naval Reserve. He became a Navy pilot and then a flight instructor. After three years of naval service, Enders returned to Yale to complete his undergraduate studies.

After Enders graduated from Yale he tried his hand at selling real estate in Hartford. However, selling real estate troubled him, in part because he believed that people ought to know whether or not they wanted to buy a house, rather than needing to be sold (2, 3). Thus, Enders considered other callings, finally deciding to prepare for a career teaching English literature.

What might have motivated that particular choice? Here is one possibility. During the years when Enders was growing up in West Hartford, his father handled the financial affairs of several celebrated New England writers, including Mark Twain. [The young Enders always admired Twain’s immaculate white suits whenever he visited the Enders home (3).] So, perhaps Enders’ early exposure to eminent writers among his father’s clients planted the seed for his interest in literature. In any case, Enders enrolled at Harvard to pursue graduate studies in preparation for his new calling.

Enders received his M.A. degree in English Literature from Harvard in 1922. Moreover, he was making substantial progress towards his Ph.D., when his career took yet another rather dramatic turn; one reminiscent of that taken later by Harold Varmus, who likewise did graduate studies in English literature at Harvard, with the intent of becoming an English teacher (4).

The changes in the career plans of both Enders and Varmus—from teaching English literature to biomedical research—were prompted by the friends each had who were at Harvard Medical School. Varmus’ friends were his former classmates from Amherst College. Enders first met his friends from among his fellow boarders at his Brookline rooming house.

Dr. Hugh Ward, an instructor in Harvard’s Department of Bacteriology and Immunology, was one of the friends Enders met at his rooming house. Enders wrote, “We soon became friends, and thus I fell into the habit of going to the laboratory with him in the evening and watching him work (5).” Enders was singularly impressed by Ward’s enthusiasm for his research (5).

During one of the trips that Ward and Enders made to the laboratory, Ward introduced Enders to Hans Zinsser, Head of Harvard’s Department of Bacteriology and Immunology. Zinsser was an eminent microbiologist, best known for isolating the typhus bacterium and for developing a vaccine against it.

Enders soon became fascinated by the research in Zinsser’s lab. So, at 30-years-of-age, and on the verge of completing his Ph.D. in English Literature, Enders changed career plans once again; this time to begin studies toward a doctorate in bacteriology and immunology, under Zinsser’s mentorship.

Zinsser, a distinguished microbiologist, was also a sufficiently accomplished poet to have some of his verses published in The Atlantic Monthly. That aspect of Zinsser likely impressed the literate Enders, who described his mentor as: “A man of superlative energy. Literature, politics, history, and science-all he discussed with spontaneity and without self-consciousness. Everything was illuminated by an apt allusion drawn from the most diverse sources, or by a witty tale. Voltaire seemed just around the corner, and Laurence Sterne upon the stair. . . . Under such influences, the laboratory became much more than a place just to work and teach; it became a way of life (3).”

Enders was awarded his Ph.D. in Bacteriology and Immunology in 1930. Afterwards, he remained at Harvard, as a member of the teaching staff, until 1946, when he established his own laboratory at the Children’s Medical Center in Boston.

Why might Enders have been satisfied staying so long at Harvard, for the most part as Zinsser’s underling? Perhaps that too might be explained by his financial independence. In any case, in 1939, while Enders was still at Harvard, he initiated the singularly significant course of research for which he is best remembered.

In 1939, in collaboration with Dr. Alto Feller and Thomas Weller (then a senior medical student), Enders began to develop procedures to propagate vaccinia virus in cell culture. After achieving that goal, the Enders team applied their cell culture procedures to propagate other viruses, including influenza and mumps viruses.

Enders and his coworkers were not the first researchers to grow viruses in cell culture. However, they were the first to do so consistently and routinely. Thus, the Enders lab launched the “modern” era of virus research in vitro. Virology could now advance much more quickly than before, since most virologists would no longer need to grow, or study their viruses only in live animals.

A recurrent theme on the blog is that key scientific discoveries may well be serendipitous. The case in point here was the unforeseen 1949 discovery by Enders and his young collaborators, Tom Weller and Frederick Robbins, that poliovirus could be grown in cultured cells. That crucial discovery made it possible for Jonas Salk and Albert Sabin to generate a virtually unlimited amount of poliovirus and, thus, to create their polio vaccines. Importantly, the discovery happened at a time when polio researchers believed that poliovirus could grow only in nerve cells. Their dilemma was that nerve cells could not be cultured in the laboratory.

Enders, Weller, and Robbins were not working on polio, nor did they have any immediate intention of working on polio when they made their finding. In fact, when the thirty-year-old Robbins (see Aside 1) came to work with Enders, he proclaimed that he wanted to work on any virus, except polio (6).

[Aside 1: Weller was one year older than Robbins. Both had been Army bacteriologists during the Second World War, and they were classmates and roommates at Harvard Medical School when they came to Enders for research experience. Robbins’ father-in-law, John Northrop, shared the 1946 Nobel Prize in chemistry with James Sumner and Wendell Stanley (7). In 1954, Robbins joined his father-in-law as a Nobel laureate (see below).]

The Enders team was trying to grow varicella (the chicken pox virus) when, on a whim; they made their critical discovery. It happened as follows. While attempting to propagate varicella virus in a mixed culture of human embryonic skin and muscle cells, they happened to have some extra flasks of the cell cultures at hand. And, since they also had a sample of poliovirus nearby in their lab storage cabinet; they just happened to inoculate the extra cell cultures with polio virus.

The poliovirus-infected cultures were incubated for twenty days, with three changes of media. Then, Enders, Weller, and Robbins asked whether highly diluted extracts of the cultures might induce paralysis in their test mice. When those highly diluted extracts indeed caused paralysis in the mice, they knew that poliovirus had grown in the cultures. See Aside 2.

[Aside 2: Whereas Enders, Weller, and Robbins did not have pressing plans to test whether poliovirus might grow in non-neuronal cells, they probably were aware of already available evidence that poliovirus might not be strictly neurotropic. For instance, large amounts of poliovirus had been found in the gastrointestinal tract.]

Despite the exceptional significance of their discovery, Robbins said, “It was all very simple (6).” Weller referred to the discovery as a “fortuitous circumstance (6).” Enders said, “I guess we were foolish (6)”—rather modest words from a scholar of language and literature. See Aside 3.

[Aside 3: Current researchers and students might note that Enders’ entire research budget amounted to a grand total of two hundred dollars per year! The lab did not have a technician, and Weller and Robbins spent much of their time preparing cells, media, and reagents, as well as washing, plugging, and sterilizing their glassware.]

In 1954, Enders, Weller, and Robbins were awarded the Nobel Prize for Physiology or Medicine for their polio discovery. Interestingly, they were the only polio researchers to receive the Nobel award. The more famous Salk and Sabin never received that honor (8).

If Enders were so inclined, might he have produced a polio vaccine before Salk and Sabin? Weller and Robbins wanted to pursue the vaccine project, and Enders agreed that they had the means to do so. In fact, Weller actually had generated attenuated poliovirus strains by long-term propagation of the virus in culture; a first step in the development of a vaccine (3). Yet for reasons that are not clear, Enders counseled his enthusiastic young colleagues to resist the temptation (6). See Aside 4.

[Aside 4: Enders may have spared Weller and Robbins the sort of anguish that Salk experienced when some of his killed vaccine lots, which contained incompletely inactivated poliovirus, caused paralytic poliomyelitis in some 260 children (8).]

The Enders poliovirus group began to disperse, beginning in 1952 when Robbins became a professor of pediatrics at Western Reserve. Weller left in 1954 to become chairman of the Department of Tropical Public Health at Harvard.

Regardless of whether Enders might have regretted not pursuing the polio vaccine, he soon would play a hands-on role in the development of the measles vaccine. The first critical step in that project occurred in1954, at the time when the Salk polio vaccine was undergoing field trials. It was then that Enders and a new young coworker, pediatric resident Thomas Peebles (Aside 5), succeeded in cultivating measles virus in cell culture for the first time.

[Aside 5: Enders was known for nurturing bright young investigators. His latest protégé, Tom Peebles, spent four years in the Navy, as a pilot, before enrolling at Harvard Medical School. Peebles graduated from medical school in 1951, and then did an internship at Mass General, before coming to the Enders lab to do research on infectious diseases in children. When Enders suggested to Peebles that he might try working on measles, Peebles eagerly accepted.]

Here is a piece of the measles vaccine story that happened before Peebles’ success growing the virus in cell culture. At the very start of the vaccine project, Enders and Peebles were stymied in their attempts to get hold of a sample of measles virus to work with. Their quest for the virus began with Peebles searching the Enders laboratory freezers for a sample. Finding none there, Peebles next inquired at Boston area health centers; still without success. After several more months of fruitless searching, Peebles received an unexpected phone call from the school physician at the Fay School (a private boarding school for Boys in a Boston suburb), telling him about a measles outbreak at the school. Peebles immediately rushed to the school, where he took throat swabs, as well as blood and stool samples from several of the school’s young patients. He then rushed back to the Enders laboratory, where he immediately inoculated human infant kidney cells with his samples. [Enders obtained the cells from a pediatric neurosurgeon colleague, who treated hydrocephalus in infants by excising a kidney, and shunting cerebrospinal fluid directly to the urethra.]

Peebles monitored the inoculated kidney cell cultures for the next several weeks, hoping for a sign of a virus replicating in them. Seeing no such indication of a virus in the cultures, Peebles made a second trip to the Fay School, which, like the first trip, was unproductive.

On a third trip to the school, Peebles obtained a sample from an 11-year-old boy, David Edmonston. The sample from young Edmonston indeed seemed to affect the kidney cell cultures. Still, Peebles needed to carry out several additional experiments before he could convince a skeptical Enders and Weller—first, that a virus had replicated in the cultures and, second, that it was measles. Peebles convinced the two doubters by demonstrating that serum from each of twelve convalescing measles patients prevented the virus from causing cytopathic effects in the inoculated cell cultures. That is, the convalescent serum neutralized the virus. The measles virus growing in those cultures was named for its source. It is the now famous Edmonston strain.

Enders, in collaboration with Drs.Milan Milovanovic and Anna Mitus, next showed that the Edmonston strain could be propagated in chick embryos (3). Then, working with Dr. Samuel Katz (1), Enders showed that the egg-adapted virus could be propagated in chicken cell cultures.

By 1958, Enders, Katz, and Dr. Donald Medearis showed that the Edmonston measles virus could be attenuated by propagating it in chicken cells. Moreover, the attenuated virus produced immunity in monkeys, while not causing disease (3). Thus, the attenuated Edmonston strain became the first measles vaccine. [Tests of the vaccine in humans led to the episode at the Fernald School (1).]

The Enders measles vaccine was attenuated further by Maurice Hilleman at Merck (9). In 1971 it was incorporated into the Merck MMR combination vaccine against measles, mumps, and rubella (9, 10).

The MMR vaccine has had a remarkable safety record, and it was widely accepted until 1997; the time when the now discredited claim that the vaccine is linked to autism first emerged (10). However, even prior to the MMR/autism controversy, vaccine non-compliance was already a problem. But, in that earlier time, parents were declining to have their children vaccinated, not because of safety issues, but rather because they questioned the severity of measles. Ironically, that was why David Edmonston refused to have his own son receive the vaccine.

Despite receiving the Nobel Prize for his polio work, Enders maintained that developing the measles vaccine was more personally satisfying to him and more socially significant (3).


  1. Vaccine Research Using Children, Posted on the blog July 7, 2016.
  2. John F. Enders-Biographical, The Nobel Prize in Physiology or Medicine 1954. From Nobel Lectures, Physiology or Medicine 1942-1962, Elsevier Publishing Company, Amsterdam, 1964.
  3. Weller TH, Robbins FC, John Franklin Enders 1897-1995, A Biographical Memoir www.nasonline.org/publications/…/endersjohn.pdf [An excellent review of Enders’ life and career.]
  4. Harold Varmus: From English Literature Major to Nobel Prize-Winning Cancer Researcher, Posted on the blog January 5, 2016.
  5. John F. Enders, “Personal recollections of Dr. Hugh Ward,” Australian Journal of Experimental Biology 41:(1963):381-84. [This is the source of the quotation in the text. I found it in reference 3.]
  6. Greer Williams, Virus Hunters, Alfred A. Knopf, 1960.
  7. Wendell Stanley: First to Crystallize a Virus, Posted on the blog April 23, 2015.
  8. .Jonas Salk and Albert Sabin: One of the Great Rivalries of Medical Science, Posed on the blog March 27, 2014.
  9.  Maurice Hilleman: Unsung Giant of Vaccinology, Posted on the blog April 24, 2014.
  10.  Andrew Wakefield and the Measles Vaccine Controversy, Posted on the blog February 9, 2015.