Polio (poliomyelitis)

Polio is the only illness in the immunisation schedule that is close to being eradicated worldwide – and at the same time the one where letting up now would be most expensive. For pregnant women it is relevant for one concrete reason: it is often contained in the same vaccine that is recommended in weeks 27 to 36.

What poliomyelitis is

Polio is caused by polioviruses, of which there are three types. They are transmitted by the faecal-oral route – through contaminated hands, water or food – and to a lesser extent by droplets.

The virus first multiplies in the throat and the intestine. In the great majority of those infected it goes no further: about 95 in 100 notice nothing at all, and another four or five have flu-like symptoms for a few days with fever, sore throat and diarrhoea.

In about one in a hundred to two hundred of those infected the virus advances into the nervous system and destroys the nerve cells that control the muscles. Because these cells do not renew themselves, the paralysis persists. It is typically asymmetrical, affects the legs more often than the arms, and sensation is preserved – those affected feel everything but cannot move.

If the muscles of respiration are affected, the illness becomes life-threatening. The iron lungs of the 1950s, in which people were ventilated for months or years, are the image that has remained of this illness.

Post-polio syndrome

Some of those who came through polio and recovered develop new symptoms decades later: increasing muscle weakness, rapid exhaustion, muscle and joint pain, sometimes difficulty swallowing and breathing.

The cause is not a fresh infection. After the illness, surviving nerve cells take over the supply of additional muscle fibres – a compensation that holds for decades and then becomes exhausted. Up to sixty per cent of those formerly ill are affected.

For advice today this is an argument that reaches beyond the acute illness: the consequences of poliomyelitis do not end with recovery.

How far eradication has come

In 1988 there were more than 350,000 cases worldwide each year. Since then a global programme has reduced the number by more than 99 per cent.

Two of the three virus types have been declared eradicated: type 2 in 2015 and type 3 in 2019. The remaining wild type 1 now circulates in only a few regions, above all in Afghanistan and Pakistan.

Europe has been certified polio-free since 2002. The last case acquired in Austria goes back decades.

There is, however, a qualification that makes the picture more complicated – and it explains why one particular vaccine is used in Europe and another is not.

Oral vaccine and injection: why the change was made

There are two kinds of polio vaccine, and their history is instructive.

The oral vaccine developed by Sabin contains attenuated, live viruses. It is cheap, easy to administer and additionally produces immunity in the intestine – vaccinated children no longer excrete the wild virus and do not pass it on. It was precisely this property that made it the tool of worldwide eradication.

But it comes at a price. In very rare cases – about once in two to three million doses – the vaccine virus itself causes paralysis. And in regions with low vaccination coverage it can circulate over a longer period and in doing so mutate back to a form that once again causes paralysis.

The inactivated vaccine developed by Salk contains killed viruses and is injected. It cannot cause paralysis and protects the vaccinated person reliably, but it only partly prevents excretion of the virus in the intestine.

In Europe only the inactivated vaccine has been used since the 1990s. The reasoning behind this is clear: where no wild virus circulates any more, every case of vaccine-related paralysis is one too many. In the remaining endemic areas, by contrast, the oral vaccine is still given, because there it is the interruption of chains of transmission that counts.

The polio component in the vaccination given in pregnancy

The whooping cough vaccine recommended in Austria for pregnant women is a combination preparation. Often it is a four-in-one vaccine which, besides diphtheria, tetanus and whooping cough, also contains a polio component – abbreviated dTpa-IPV, where IPV stands for the inactivated polio vaccine.

This component too is an inactivated vaccine and is safe in pregnancy. That is no small matter: the live vaccine would not be, and in countries where it is still used a different recommendation applies to pregnant women.

Whether you receive a three- or a four-in-one vaccine is decided by availability. For the purpose – protecting the newborn against whooping cough – the two are equivalent; see whooping cough.

The immunisation schedule in Austria

For children the polio vaccination is part of the six-in-one vaccination in the first year of life, with doses in the third and fifth month of life and between the twelfth and the fourteenth. A booster follows at primary school age.

For adults a booster every ten years is provided for where there is an increased risk – above all for travel to areas where the virus still occurs, and for occupational contact. For the general population in Europe a regular booster beyond the childhood vaccinations is not strictly required; in practice it often coincides with a dTpa-IPV booster anyway.

Anyone who has never had a basic course needs three doses. An interrupted series is continued and not begun again – even after many years every dose already given still counts.

Travel

Before travelling to countries with continuing circulation of polio, or with outbreaks caused by circulating vaccine viruses, current protection is required – in part formally, with an entry in the international certificate of vaccination which some countries check on departure.

The list of countries concerned changes; it is continually updated by the World Health Organization. Before a long-distance journey it should be checked, and early enough for a vaccination still to be fitted in.

For pregnant women, what applies to every long-distance journey applies in addition: the most favourable period is the second trimester, airlines set their own limits from week 36, and medical care at the destination should be considered beforehand.

Why vaccination continues

The obvious question about an almost eradicated illness is why one cannot stop. The answer has three parts.

First, the illness is not gone as long as it circulates somewhere. A flight takes hours; an imported virus finds a foothold immediately in a poorly vaccinated population. That is exactly what happened several times in recent years – poliovirus was detected in the waste water of several European and North American cities, and in individual cases unvaccinated people fell ill.

Second, detection in waste water is the early warning system on which surveillance rests. Because only one in a hundred of those infected develops paralysis, a single case of illness means that hundreds of people are infected – the illness is the tip of an iceberg.

Third, there is no treatment. What has been destroyed in nerve cells stays destroyed. What is treated is the consequence – with physiotherapy, aids, and in the acute case with ventilation – not the cause.

Why polio became dangerous only with better hygiene

A connection that explains the history of this illness and regularly comes as a surprise in consultations: poliomyelitis became a major threat in Europe and North America precisely when hygienic conditions improved.

The reason lies in the interplay between the age at infection and protection carried over from the mother. Under poor hygienic conditions children were infected in the first year of life, at a time when they still had maternal antibodies. The infection then almost always passed unnoticed and left lifelong protection. With better water and sewage supply the age at infection shifted later – into a time without that borrowed protection – and the illness became visible.

From this arose the great epidemics of the first half of the twentieth century, which filled children's wards and marked whole birth cohorts. The German name “Kinderlähmung” (childhood paralysis) dates from that time; it is medically misleading, because adults fall ill too, and more severely.

This pattern – an illness becoming more dangerous through a shift to a later age – occurs repeatedly in infectious disease medicine; it applies to rubella and mumps as well. It is the reason why half-hearted immunisation programmes can be worse than consistent ones.

What is read from the waste water

Because ninety-nine out of a hundred infections run their course without paralysis, counting people who fall ill is no use as an early warning system. Its place is taken by the examination of waste water, in which excreted viruses can be detected long before a single case of illness occurs.

This method has been triggered several times in recent years – in London, in New York, in several European cities. In most cases these were vaccine viruses imported from countries using the oral vaccine, circulating in poorly vaccinated population groups. In one case an unvaccinated young man fell ill.

For advice, a sober sentence follows from this: the illness is not gone, it is merely not visible. And the protection that keeps it invisible is the same one handed out at every vaccination appointment.

Polio in pregnancy

Historical studies from the era of the great epidemics show that pregnant women developed a paralytic form of the illness more often than other adults, and that the rate of miscarriage and preterm birth was increased. The virus itself does not cause malformations.

This no longer has practical significance in Europe today. Where it does have significance is travel: an unprotected pregnant woman in an endemic area is an avoidable situation, and avoiding it consists of a booster before departure.

If a woman falls ill shortly before the birth, the virus can be transmitted to the child – in newborns poliomyelitis runs a particularly severe course. Here too the rule holds: in Europe this is a historical situation, but worldwide it is one of the reasons why vaccination of pregnant women in endemic areas is expressly recommended and is given there with the inactivated vaccine.

How the diagnosis is made

In any flaccid paralysis that appears within a few days and has no other identifiable cause, poliomyelitis is considered – that is the rule worldwide and the basis of surveillance. Two stool samples at least 24 hours apart are examined, because the virus can be detected there most reliably, supplemented by a throat swab and cerebrospinal fluid.

If a poliovirus is found, a genetic examination follows: it establishes whether it is a wild virus, a vaccine strain or a circulating virus derived from a vaccine strain. What measures follow depends on this distinction – and whether a country keeps its polio-free status.

The illness is chiefly distinguished from Guillain-Barré syndrome, which likewise involves rapidly progressive paralysis but runs a symmetrical course and is accompanied by disturbances of sensation. Other enteroviruses can also produce a similar picture.

Tolerability

The inactivated vaccine is well tolerated. The commonest reactions are local – redness, swelling and pain at the injection site for one to two days – together with occasional fatigue or slight fever.

The vaccine contains traces of the antibiotics used in its manufacture – usually neomycin, streptomycin and polymyxin B. A known severe allergy to these is one of the few reasons not to vaccinate; an ordinary contact allergy to one of these substances is not. An acute febrile illness leads to postponement, not to abandonment.

Because in Austria the polio component is almost always given as part of a combination vaccine, the pattern of reactions corresponds to what is known from the dTpa vaccination: local reactions common, general reactions occasional, severe reactions very rare.

What makes eradication so difficult

After smallpox, polio was to become the second illness to disappear completely. The target has been postponed several times, and the reasons for that are revealing – almost all of them lie outside medicine.

The silent infections. With smallpox every infected person was visible; you could vaccinate around each case and break the chain. With polio the great majority is invisible, which makes the same strategy impossible.

The vaccine viruses. The oral vaccine, without which eradication would never have come this far, itself produces circulating viruses. The switch to the inactivated vaccine, which solves this problem, is more expensive and organisationally more demanding – it needs syringes and trained staff instead of two drops.

Conflict and mistrust. The remaining regions are those in which vaccination teams cannot work safely. In several countries vaccinators have been killed; rumours about the intentions behind the programmes have cost years.

The lesson from this is uncomfortable and deserves saying nonetheless: immunisation programmes rarely fail because of the effectiveness of the vaccines. They fail on accessibility, on trust and on persistence. For the individual woman facing the question of whether to take a booster in pregnancy that changes nothing – but it puts into context why this vaccination is still in the schedule after fifty years.

What happens next

In this practice the vaccination in pregnancy is discussed and given – as a three- or four-in-one vaccine depending on availability. Please bring your vaccination record; it will also show whether a polio booster would be due anyway. Arranging an appointment.

If you are planning a journey, say so when you make the appointment. Travel vaccinations have lead times – some need several doses over weeks – and in pregnancy the question of which of them are possible at all comes on top. That clarification belongs at the start of planning a journey and not in the week before it.