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Poliovirus in 2022: Issues for Pregnancy

From reading The Pulse, from outside reading, and from talking with your doctor, you may know that poliomyelitis —polio for short— is a viral condition against which children have been vaccinated since the 1950s. In the event that you were not vaccinated against polio during childhood, or not fully vaccinated as you missed one or more dose, polio vaccination is available while you are an adult, including during pregnancy. This is extremely important for reasons that we’ll discuss. From the news, you also may have heard that health investigators detected poliovirus, specifically vaccine-derived poliovirus type 2 (VDPV2), in the London sewage system back in June. You also may have heard that in July, there was a case of actual polio in Rockland county, New York, whose childhood vaccination rate against polio is only 42 percent. The low vaccination rate stems from the fact that Rockland county has the highest population of ultra-orthodox (Haredi) Jews, whose vaccination rate itself is low, and indeed the case of polio occurred in a member of one of the Haredi communities, a 20-year-old man. Not long after the patient fell sick, his virus was identified as VDPV2, the type of poliovirus detected the previous month in the London sewage system and news came out that the patient had been traveling in Poland and Hungary in the weeks prior to falling ill. All of this raises the question of whether there is an outbreak of polio that should concern you, particularly if you are pregnant.

The first thing to know is that, if you received your childhood polio vaccination, consisting of multiple doses of polio vaccine and if you are not in any situation that would make you immunocompromised, you are in good shape. If there is some issue that your doctor thinks might compromise your immune system, you can receive a booster dose of the inactivated polio vaccine (IPV). This is the only polio vaccine now offered in most developed countries, such as the United States and the United Kingdom and it the kind that you can receive while you are pregnant. This is in contrast with the oral polio vaccine (OPV), the live attenuated vaccine, which the US stopped using in the year 2000 and actually is part of the reason for the finding in the London sewers and the case in Rockland county, although that case would not have occurred, had the man been vaccinated with IPV.

Before delving into the difference between IPV and OPV, let’s talk about what contracting polio would mean for an unvaccinated woman who is pregnant, which means first let’s talk about the basics of polio. There are three groups of polioviruses, PV1, PV2, and PV3. They are enteroviruses, meaning that they infect the gastrointestinal tract. In nearly all cases, the virus just stays in the gastrointestinal tract until the immune system gets rid of it, and while the virus is in there, the person sheds the virus in stool and can infect others. This can be from infection from the anus or mouth, which can include saliva transmitted to another’s mouth by talking. In nearly all cases, the virus doesn’t do anything other than stay in the gastrointestinal tract and infect others, but in some cases it infects the central nervous system (CNS), killing neurons (nerve cells) in a very particular part of the CNS, called the anterior horns of the spinal cord. This is where motor function is controlled, including control of voluntary muscles and control of breathing. After an initial period of flu-like symptoms, the victim may suffer paralysis, especially of limbs as many of the neurons that control muscles, such as limb muscles, die. Often, this leaves just a small fraction of such neurons working and they try to take over the function of neighboring neurons that died. This results in flaccid paralysis, the kind of paralysis that leaves muscles loose and limbs floppy, leading to the term “floppy baby syndrome”. Basically it’s extreme weakness, which can worsen as the person ages, because the motor neurons that did survive and that took over the function of others, gradually wear out over many decades. Because breathing muscles, including the main breathing muscle, the diaphragm, are affected, this is why victims of polio were treated in iron lungs in the mid 20th century, when polio epidemics reached a peak. Actually, the US had suffered a major polio outbreak in 1916, then polio had returned in various outbreaks that struck during summer in the following decades, leading to a still larger outbreak in the early 1950s.

During the mid 20th century polio outbreaks, some data were recorded from pregnant women with the disease. Notably, there were 75 cases of polio in pregnant women in Minnesota in 1946, when there was a concerted effort to take note of what happened to the pregnancies and publish the findings in 1951. In the realm of good news, researchers found that pregnancy did not make the course of the disease worse in women who did contract the disease. Recall our discussion here on The Pulse of COVID-19 and how the disease tends to be worse in pregnant women compared with non-pregnant women, but that’s not the case with polio, although for survivors of polio who suffer paralysis, there are issues if they become pregnant later. Notably, if the legs are affected in a growing child, her pelvis may not develop properly and she may not have enough control or endurance to push with her abdominal muscles during labor. These issues could make it more likely that she’ll need a cesarean section. But the study of the 75 women also brought bad news. Notably, the attack rate of the virus was found to be higher in pregnant women, meaning that being pregnant makes you more susceptible to developing polio in the event that you are exposed to the virus. Back then, researchers discussed the timing of pregnancy in terms of months, rather than weeks, and they noted that the more severe cases of polio tended to strike pregnant women from the second to the fifth month, so that means from about week 5 through week 22 of pregnancy the way we count it today. In other words, you’re most vulnerable during the first half of pregnancy. The researchers also noted a slight dominance of male fetus among those pregnant women with polio, especially the more severe cases, but it’s not clear if that was a significant finding. The researchers did not any increase in the rate of congenital defects.

As for what this all means, we it means that you should be vaccinated and if you are pregnant and are not sure if you completed your childhood polio vaccination series, you could get a booster of IPV. This brings up the difference between the two types of vaccines, so let’s move to that topic now. IPV is an inactivated or “killed” vaccine, meaning that it cannot reproduce in your body. This is the vaccine developed by Jonas Salk in the 1950s. It is injected into your arm and all that it does is to provoke an immune response against the three types of polioviruses, should you ever be exposed to them. It actually does not prevent you from getting infected with a poliovirus in your gastrointestinal tract, but it does prevent you from getting sick, even if you do get infected in your gastrointestinal tract. Because IPV is inactivated, there is no concern about it doing any harm to the fetus, or embryo.

In contrast, OPV, the vaccine developed by Albert Sabin in the early 1960s, is a live attenuated vaccine, the type of vaccine that is not given to pregnant women. Advantages of OPV are that it is cheaper and easier to distribute than IPV, plus it protects the gastrointestinal tract from infection, so it is still used in countries where vaccination rates are relatively low and it would be difficult and expensive to deliver IPV. It’s not even injected. It’s a liquid that the person drinks, so it doesn’t even require trained nurses to administer it. The way that OPV works is that processing of the virus turns off certain genes that make the virus potent, so that all that the altered virus does is to immunize and reproduce in the gastrointestinal tract of the recipient, plus it can immunize others who get exposed to the vaccine from a recipient. This means that the OPV can spread through a population, even when not everybody receives it directly. On rare occasions, however, one or more of the genes that were turned off can turn back on. This can cause some level of polio disease, especially in a person who is immunocompromised. Beyond this scenario, an attenuated polio virus that has reverted back to a disease causing polio virus can spread through sewage and similar routes. This is called a vaccine-derived poliovirus and it happens with PV2, which is in the OPV in attenuated form. That’s where we get the term vaccine-derived poliovirus type 2 (VDPV2) that gave the Haredi man polio in Rockland county.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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