What is a “vaccine-derived” poliovirus?

New York health officials announced in July that an unvaccinated adult man in Rockland County was diagnosed with polio. This is the first case of life-threatening illness in the United States since 2013. The virus that causes polio was later detected in New York City wastewater. , and city and state health officials now say the virus is likely circulating in the city.

The virus confirmed in New York is a vaccine-derived poliovirus. Wild poliovirus has been eradicated from most of the world and is currently endemic only in Afghanistan and Pakistan.

However, vaccine-derived viruses can still cause outbreaks, emerging when the attenuated virus in the oral polio vaccine mutates and spreads to unvaccinated populations.

VOA spoke with three polio experts about vaccine-derived poliovirus and oral polio vaccine. Here’s what you should know:

What is vaccine-derived poliovirus?

Vaccine-derived poliovirus is related to the active virus in the oral polio vaccine (OPV).

OPV works by infecting cells in the gut with a weakened poliovirus, allowing the body to safely immunize against polio without the risk of being paralyzed by the actual disease.

“[The weakened viruses] still get infected. They still replicate in your gut. It gives you lifelong immunity, but it doesn’t paralyze you,” said virologist Konstantin Chumakov, director of the Global Virus Network Center of Excellence and adjunct professor at George Washington University.

“However [vaccine-derived] the virus is still contagious [from person to person]” added Chumakov. “This was seen as a great advantage of the vaccine, basically because one dose can immunize several children.”

This transmission is a problem in areas with low vaccination coverage. If the virus can spread for a long period of time, it is more likely to mutate back to its dangerously paralytic form.

Why is the oral polio vaccine (OPV) used?

Although OPV is related to the vaccine-derived poliovirus, it has many advantages over inactivated polio vaccine (IPV), the injectable vaccine that is still used in most of the world.

An OPV recipient cannot be a “silent spreader”. After an immune response to the vaccine develops, they have immunity to polio for the rest of their lives.

In contrast, IPV prevents paralysis but does not prevent the virus from replicating in the gut. People who get IPV can spread polio even though they don’t get sick.

“The ‘gut immunity’ that OPV provides is essential in populations that want to stop the spread,” said Capt. Derek Erhardt, polio incident manager at the Centers for Disease Control and Prevention (CDC). but can still spread [paralytic polio] Neighbors, brothers, sisters, anyone. ”

In doing so, OPV has helped eradicate wild polio in most parts of the world, including countries such as the United States, which currently use only IPV.

OPV has other advantages as well.

“We are using a low-cost oral polio vaccine,” said Richter Razafindratsimandresi, director of the National Reference Institute for Polio at the Madagascar Pasteur Institute. , is easy to administer, there is no need to inject the child, and it is not a problem for parents to accept.”

If OPV can prevent silent spread, why are the US and other countries using only IPV?

After the United States and other wealthy nations eradicated wild polio, they stopped using OPV. This is because there is a small risk of creating vaccine-derived poliovirus in under-vaccinated communities, causing paralysis. According to the Global Polio Eradication Initiative (GPEI), this occurs about 2-4 times per million live births. In the United States, the risk is now considered unacceptable because the OPV campaign has been so successful that there have been very few cases.

The United States and other wealthy nations have robust healthcare systems that can effectively acquire the more expensive IPV and introduce it into all sectors. The use of IPV therefore suggests that well-vaccinated populations in these countries are adequately protected from polio-induced paralysis despite poor protection against poliovirus infection and spread. means that there is

How does vaccine-derived poliovirus cause outbreaks?

Vaccine-borne poliovirus outbreaks are more likely in countries with weak immunization systems. It happened several times in Madagascar, he said, Razafindratsimandresy.

“[The virus] I am not from another country.Because Madagascar has a range of immunizations for… [that] Very, very low,” he said.

Poliovirus from imported vaccines can cause outbreaks in countries with high vaccination coverage, especially if IPV is used instead of OPV.

“Essentially, everyone is protected from paralysis, so it can be passed from person to person without causing symptoms,” Chumakov said. It can attack an unvaccinated or immunocompromised person and paralyze them, which is exactly what happened in New York.”

Are OPVs safe?

“Our vaccine is safe and effective,” said Erhardt. “We need to vaccinate unvaccinated children to stop the ongoing spread of these viruses.”

Chumakov said developing a better version of OPV could reduce the risks while maintaining the intestinal immunity provided by OPV. He was previously involved in his GPEI effort to develop a safer oral vaccine against type 2 poliovirus. Clinical trial data suggests that the new His OPV is less likely to revert to its dangerous form.

Vaccines may improve in the future, but vaccinating as many children as possible with pre-existing OPV remains a priority for many in the world.

According to the CDC, global polio vaccination coverage will drop to 81% in 2021, the lowest in a decade. This is largely due to his COVID-19 pandemic, but not all countries have sufficient resources for vaccination campaigns even in peacetime.

Razafindratsimandresy said that despite Madagascar aiming to vaccinate all children against OPV, the country does not have enough personnel and often lacks vaccines.

“To stop these diseases, we need to close these immune gaps,” said Erhardt. “Children around the world are still at risk if they have polio anywhere.”