The two vaccines that brought us to the brink of polio eradication | Explanation

The two vaccines that brought us to the brink of polio eradication | Explanation

In 1948, microbiologists John F. Enders, Thomas Weller, and Frederick Robbins were trying to find a way to grow different viruses in cell culture. In a routine experiment involving human muscle and skin cells, they decided to test another virus along with the virus already being tested, because a vial containing that virus was in their freezer. To their surprise, the virus grew and evolved well with their method. Their work ultimately solved one of the most important scientific problems of the time.

They just managed to find a way Spreading the polio virus in non-neuronal cells.

A major obstacle

In the mid-20th century, researchers widely believed that poliovirus could only be grown in cultures of nerve cells. This misconception was fueled by their inability to infect rhesus macaques by the oral route and only by injecting the virus directly into the nervous system. At the time, they did not know that the problem was in the poliovirus strains they were using.

Poliovirus has only one natural host – humans – and many of the early strains of the virus were isolated from humans and would not infect non-human primates. Because scientists continued to pass the virus through the brain tissue of macaques, it adapted to that method of infection.

The inability to grow polio in non-nervous cells was a major hurdle in developing a polio vaccine. But thanks to Enders and his team, poliovirus can now be mass-produced for vaccine research.

Elimination target missed

Polio eradication is one of the top priorities of the World Health Organization (WHO). Since Africa was declared polio-free in August 2020, the wild poliovirus has been confined to rural areas of Afghanistan and Pakistan. But even here, according to a report recent report In ScienceThe virus has started appearing again in big cities of both these countries.

This resurgence is a result of vaccine hesitancy due to misinformation, conflict, poverty and limited access to these isolated areas. The WHO global polio eradication initiative is thus set to miss its deadline of eradicating polio by the end of 2024.

The failure of this initiative in Afghanistan and Pakistan casts a long shadow over a remarkable achievement: the eradication of polio everywhere. This achievement was achieved by two vaccines, both invented within a year of each other. They are administered differently, provide different levels of protection, contain different components, and target different parts of the immune system. Yet both played equally invaluable roles in the global fight against polio.

Systemic and mucosal

In late 1949, Enders received a letter asking for the starting material and procedure his team had developed for growing poliovirus in culture. At the time, Enders and his team were discussing their future work. While his young colleagues Weller and Robbins wanted to use the method to advance vaccine research, Enders said such work was unsuitable for basic science researchers like him. He handed the sample and procedure over to the letter's author, Jonas Salk, director of the Virus Research Laboratory at the University of Pittsburgh.

Salk created the first successful vaccine for polio. He grew the virus using the method developed by Enders and his team, inactivated it by treating it with formaldehyde, and injected it into his test subjects. The pieces of the inactivated virus were able to generate immunity in their bodies. Importantly, since the vaccine was injected into the muscle, it generated systemic immunity.

The immune system has two main parts: systemic and mucosal. The systemic component includes the blood, brain, and all other organ systems. The mucosal component includes the digestive and respiratory tracts, the urogenital tract, and the inner linings of the eyes. These areas are lined with mucous membranes that provide an additional layer of protection, since they are frequently exposed to the outside environment.

Attack on Ground Zero

Soon after Salk created the inactivated polio vaccine (IPV), Albert Sabin developed another vaccine that contained live polio strains that had been weakened by growing them serially in macaque cells, making them unsuitable for human infection. Since Sabin's vaccine contained live virus particles, it had to rely on its natural mode of infection and was therefore given orally. This was the oral polio vaccine (OPV).

As OPV traveled into the stomach, it induced a powerful protective mucosal immune response in exactly the place where the virus was supposed to begin its infection.

OPV had several advantages over IPV. First, the vaccine produced a protective response at the site of entry of the virus – the gut – allowing it to provide much greater protection relative to IPV. Second, OPV was given orally and did not require syringes or trained personnel for its administration.

One-Two Punch

But there was a problem. Sometimes, the weakened virus in OPV would revert, and do what it was designed to do: cause polio. IPV, on the other hand, despite being a less potent vaccine, contained inactivated virus particles and posed no risk of causing vaccine-induced polio.

The world has used both vaccines in the fight against polio. While some countries like Norway, Sweden, Finland and Iceland relied solely on IPV, most countries have used a combination of both. The latter countries prefer OPV for better safety and ease of administration. When the number of natural polio cases drops to zero, they switch to IPV for its increased protection.

Despite the many differences between Salk's and Sabin's vaccines, they have one important feature in common that empowered the WHO in the fight against polio: both Jonas Salk and Albert Sabin chose not to patent their vaccines. When asked who held the patent to his vaccine, Salk famously replied: “Well, the people, I would say. There is no patent. Can you patent the sun?”

Arun Panchapakesan is an Assistant Professor at YR Gaithonde AIDS Research and Education Centre, Chennai.

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