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Story of the drop: what's inside a polio vaccine that prevents up to 3 million deaths every year

Oral and inactivated types of vaccines use a live strain of the poliovirus to build antibodies and improve overall immunity.
Updated 31 Oct, 2020 04:26pm

Poliomyelitis, often identified as polio, is a crippling disease that remains preventable but entirely incurable around the globe. Till date, the only two vaccines available to the world for prevention against the wild virus are oral polio vaccines (OPVs) and inactivated polio vaccines (IPVs).

While some positive cases remain asymptomatic, others have reported fevers, stomach pains, sore throats, nausea, stiff necks, and headaches.

The disease is also known to travel from person to person via contaminated water.

According to the WHO, every year, immunisation campaigns help keep up to 3 million children under the age of five from dying of the disease that causes bone-related deformities.

How does the virus attack?

The live poliovirus attacks the human body by replicating itself first in the intestines; it then journeys all the way to the brain and spinal cord through blood.

Polio is known to cause paralysis when the virus is successful in reaching and attacking the nervous system.

In order to prevent this disease from leaving about 18 million people paralysed, two key vaccines were developed and made available in the 1950s; these are commonly known as inactivated polio vaccines (IPV) and oral polio vaccines (OPV).

Since discovery, the oral polio vaccine, which is a cheaper and more widely used solution in the developing world, has been administered through liquid drops via the mouth, while the inactivated polio vaccine, now the only form of polio vaccine used in the US, is given in the form of a series of shots.

What are polio vaccines really made of?

The early 1950s witnessed a remarkable advancement in the discovery of vaccines at the hands of Dr. Jonas Salk and Dr. Albert Sabin.

In what is seen as a near-miraculous milestone in the history of epidemics, these vaccines were successful in ridding nearly the whole world of the crippling virus.

To put together the oral polio vaccine (OPV), a mixture was created out of three live, attenuated poliovirus strains.

In order to keep the human body free of virus, the OPV builds antibodies in the blood against all three types of poliovirus, which shield the nervous system when an attack takes place and produce an immune response by lining the intestines, which are the first and preferred home of the poliovirus when inside the human body.

This response of the OPV makes it one of the most widely acknowledged solution to barring person-to-person transmission of the wild virus.

While the OPV was produced by weakening strains of the poliovirus, the IPV, or inactivated polio vaccine, purified the virus and then killed it with a chemical called formaldehyde.

To prevent paralysis, an IPV stops the wild poliovirus from reaching the brain and spinal cord, instead of lining the intestines.

Both methods of immunisation gained popularity due to their widespread success around most parts of the world, but the polio shot is more commonly preferred as it contains a dead virus that does not carry the ability to replicate, which is a key requirement for the disease to cause paralysis.

This widespread preference is rooted in the oral vaccine's inability to reduce side effects to zero.

It is said that in one of every 2.4 million recipients, the weakened virus strains carried in the OPV cause paralysis.

However, many countries around the world continue to use OPVs as these are relatively affordable, easy to use and good at building and maintaining immunity.

Widespread use of IPVs outside the US, in the developing world per se, has been difficult as these vaccines bring along high costs with an inability to assure biocontainment required for production.

Source: historyofvaccines.org
Source: historyofvaccines.org

It is believed that the poliovirus has continued to attack humans for millions of years. Evidence of a crippling disease causing various bone deformities that appear polio-like in nature has lived through Egyptian carvings since the 1400s.

The disease was largely uncommon before mid of 1800s.

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It was in the 1900s that the wild poliovirus gained strength and turned into an epidemic.

In the recent times, following the 2000s, polio has been reduced to an endemic with strongholds in Pakistan and Afghanistan, with much larger numbers of active cases being reported as vaccination campaigns are faced with numerous challenges due to the coronavirus pandemic.

Pakistan is currently running a countrywide immunisation campaign aiming to vaccinate 40 million children this year, out of which around 32 million have already been successfully vaccinated across 130 districts.