The alphanumeric soup known as influenza

Robert Herriman, co-founder of The Global Dispatch, recently started a radio show called Outbreak News This Week. Robert calls the show “Your source for all the news about worms and germs”. He covers the latest news and information about infectious diseases and often includes interviews with expert guests. The show can be heard live Saturday mornings at 7:30 am EST on on The Tan Talk Radio Network: 1340 AM WTAN Clearwater, 1350 AM WDCF Dade City and 1400 AM WZHR Zephyrhills. You can also listen online.

I have been a frequent guest on Robert’s Outbreak News This Week, most recently this past Saturday, when we had a broad-ranging conversation about influenza virus. We also managed to squeeze in a few words about my favorite virus, poliovirus, and India’s success in remaining polio-free for three years. Listen below.


India has been free of polio for three years

Poliovirus cutaway

Image credit: Jason Roberts

Three years ago today, on 13 January 2011, the last case of poliomyelitis was reported in India. This achievement represents a remarkable turnaround for a country where control of the disease had for years been extremely difficult. As recently as 2009 there were 741 confirmed cases of polio caused by wild-type virus in India. Being polio-free for three years is certainly a cause for celebration, but not for becoming complacent. Immunization efforts in India must not decline, because wild-type and vaccine-derived polioviruses continue to circulate and pose a threat to any unimmunized individual.

Wild polioviruses – those that have always been circulating in nature – continue to cause disease in Afghanistan and Pakistan, two countries close to India. Pakistan reported 58 polio cases in 2012, and 85 so far in 2013; for Afghanistan the numbers are 37 and 12. But distant countries can also transmit polio: recent outbreaks in the Horn of Africa and in Syria originated in Nigeria and Pakistan, respectively.

Perhaps a greater threat are vaccine-derived polioviruses. The Sabin poliovirus vaccines, which have so far been the mainstay of the polio eradication effort, comprise infectious viruses that are taken orally. Upon replication in the intestinal tract, the vaccine strains confer immunity to infection, but they also revert and become capable of causing paralysis. Such vaccine-derived polioviruses circulate and can cause outbreaks of polio. Because India has been using Sabin poliovirus vaccines intensely for many years, there is no doubt that vaccine-derived polioviruses are circulating in that country. If polio vaccine coverage drops, there will be outbreaks of polio caused by vaccine-derived strains. Even if wild polioviruses disappeared from the globe, as long as Sabin vaccines are used, vaccine-derived polioviruses will circulate. The solution to this conundrum is to switch to Salk’s inactivated poliovirus vaccine and wait for the Sabin-derived strains to disappear. This switch is now part of the WHO’s eradication plan (it wasn’t always), but it will not be easy: Salk vaccine must be injected, and therefore requires trained health care personnel; administering Sabin vaccine requires no special skills. But we cannot simply stop immunizing with Sabin vaccine – that is a recipe for outbreaks of polio.

According to the World Health Organization, being free of wild polio for three years means that the virus is probably no longer endemic in India. However, WHO does not certify individual countries as polio-free; rather it declares a WHO region polio-free when all countries in the Region have not reported a case of wild polio for 3 years in the face of highly active surveillance. The Americas, the Western Pacific, and European regions have been declared polio-free by WHO. India is part of the South-East Asia region, which also includes Bangladesh, Bhutan, Democratic People’s Republic of Korea, Indonesia, Maldives, Myanmar, Nepal, Sri Lanka, Thailand, and Timor-Leste, none of which have reported a case of polio for 3 years. WHO will decide in March whether to declare this region polio-free. That would leave the regions of Africa and the Eastern Mediterranean as the last known reservoirs of wild poliovirus.

Can India remain polio-free?

global polio 2012India has been free of polio for over one year. This is a remarkable accomplishment, considering that just 30 years ago the country recorded 200,000 cases of the disease annually, or one every three minutes. With polio endemic in two neighboring countries, Pakistan and Afghanistan, and in the more distant Nigeria (figure), can India remain free of the disease? According to Shahnaz Wazir Ali, the Pakistani Prime Minister’s focal person for polio, there is little risk for export of the virus from Pakistan:

The likeliness of polio virus being exported to India from Pakistan is very low, and historically, it has not happened. Those who travel from India to Pakistan are mostly adults. There are rarely any babies. So the chances are low.

It is correct that polio has not traveled from Pakistan to India during the modern era of virus detection (1980 to the present). However, the same genotypes of types 1 and 3 poliovirus have circulated in both countries, implying sharing of viruses some time in recent history. Therefore Ali cannot conclude that export of virus to India ‘has not happened’.

Poliovirus continues to circulate in Pakistan, which shares a border with India: there were 198 cases in that country in 2011, the most of any in the world, and 16 cases so far in 2012. Remember that most poliovirus infections are asymptomatic, so the number of paralytic cases is far lower than the actual number of infections. The ratio of paralytic cases to infections varies according to the viral serotype: 1:200, 1:1800, and 1:1200 for types 1, 2, and 3 respectively. Furthermore, poliovirus has been known to spread from Pakistan from other countries. An outbreak of polio in Xinjiang, China, in 2011 was caused by virus imported from Sindh, Pakistan. There were 21 paralytic cases caused by poliovirus type 1, over half of which occurred in individuals 19-53 years old. The outbreak was halted by immunization but the region remains at risk for importation from Pakistan.

Poliovirus also continues to circulate in Afghanistan, which lies on the northwestern border of Pakistan. Eighty cases of paralytic disease were reported in this country in 2011, and 7 so far in 2012. It has been difficult to control polio in the southern provinces of Kandahar and Helmand due to ongoing armed activities. There is active migration between the southern regions of Afghanistan and Pakistan which has lead to a steady exchange of polioviruses between the two countries.

There many other examples of polio spread from one country to another in recent years. Following cessation of polio immunization in 2003, virus spread from Nigeria to many countries in Africa as well as to Indonesia. From India poliovirus has spread to Nepal, Angola (2005 and 2007), and Tajikistan and then to Russia (2010), in all cases causing substantial outbreaks of the disease.

The message is clear: poliovirus spreads easily among countries, and it is often spread by infected adults, not children. Because poliovirus infection is frequently asymptomatic, such spread cannot be detected by simply examining travelers for signs of paralysis.

For these reasons I am skeptical of Ali’s reassurance that the virus is not likely to spread from Pakistan to India. If adults mainly travel from India to Pakistan, as she says, they could well be infected and import the virus back home before it is detected. Furthermore, adults could bring the virus to India from other countries where poliovirus continues to circulate, although that is not Pakistan’s concern.

Because Pakistan remains a major reservoir of poliovirus, it is a good sign that the country is acknowledging the possibility that they might export the virus to India. The best way to avoid this scenario would be to intensify their immunization programs and eliminate the virus. Apparently Ali has been speaking with Indian officials to learn how they accomplished this goal:

We got to know what actually took India to become polio-free. We have understood the scale and efforts that we require to make Pakistan polio-free.


India polio-free for one year

Year in polio 2011A year has passed since the last reported case of poliomyelitis in India, which occurred on 13 January 2011 in a two year old girl in Howrah, West Bengal. If no additional cases are reported in the next few weeks (some samples are currently being tested for the virus), then it will mark the first time that India has been polio free for one year.

This achievement represents a remarkable turnaround for India, where control of the disease had for years been extremely difficult. As recently as 2009 there were 741 confirmed cases of polio caused by wild-type virus (as opposed to vaccine-derived virus) in India. The tide turned in 2010 with only 42 confirmed polio cases, and in calendar year 2011 there was just one. That is why the 2011 map marking locations of confirmed wild polio cases in India (see figure) shows only one red dot (paralysis caused by type 1 poliovirus) in the country. The blue dots indicate cases caused by type 3 poliovirus.

The challenge now is to keep India free of polio. The map shows why this will be difficult – there are many red dots (cases of type 1 polio) in neighboring Pakistan and Afghanistan. Poliovirus does not respect national borders – China had been free of polio since 1999, but now there are red dots in that country. That outbreak was imported from Pakistan. Even the polio cases in more distant countries such as Africa constitute a threat. As long as there is polio somewhere, all countries must maintain extensive immunization programs. Whether or not that will happen depends upon money, determination, and allowing immunization campaigns to proceed without interruption.

Once polio was eradicated from the United States, the only poliomyelitis was caused by the Sabin vaccine. Consequently this country switched to the use of inactivated vaccine in 2000. As other countries eliminate the disease, vaccine-associated poliomyelitis will become more prominent. If eradication of polio is achieved, the world will have to switch to using inactivated poliovaccine.


Wild poliovirus in China
Dreaming of inactivated poliovaccine
Poliomyelitis after a twelve year incubation period
Poliovirus vaccine litigation


TWiV 36: Pandemic

twiv-200On episode #36 of the podcast “This Week in Virology”, Vincent, Alan, Dick, and Hamish Young discuss the 2009 influenza pandemic, first 2009 H1N1 vaccine, hunting mosquitoes with midges, vaccine-associated polio in India, and adenoviruses.

Click the arrow above to play, or right-click to download TWiV #36 or subscribe in iTunes or byemail.