Question: How many centers in the world are authorized by the WHO to keep stocks of smallpox virus?
About the article "Virus", section "Weapons".

Candidate passages:
The ability of viruses to cause devastating epidemics in human societies has led to the concern that viruses could be weaponised for biological warfare.
Further concern was raised by the successful recreation of the infamous 1918 influenza virus in a laboratory.
The smallpox virus devastated numerous societies throughout history before its eradication.
There are only two centers in the world that are authorized by the WHO to keep stocks of smallpox virus: the Vector Institute in Russia and the Centers for Disease Control and Prevention in the United States.
However fears that it may be used as a weapon may not be totally unfounded.
As the vaccine for smallpox sometimes had severe side-effects, it is no longer used routinely in any country.
Thus, much of the modern human population has almost no established resistance to smallpox, and would be vulnerable to the virus.
Smallpox was eradicated by a massive outbreak search and vaccination program started in 1967, organised and co-ordinated by a World Health Organisation (WHO) unit set up and headed by Donald Henderson.
The last case in the Americas occurred in 1971 (Brazil), south-east Asia (Indonesia) in 1972, and on the Indian subcontinent in 1975 (Bangladesh).
What, after two years' intensive searches, proved to be the last endemic case anywhere in the world occurred in Somalia in October 1977.
A Global Commission for the Certification of Smallpox Eradication chaired by Frank Fenner examined the evidence from, and visited where necessary, all countries where smallpox had been endemic.
In December 1979 they concluded that smallpox had been eradicated; a conclusion endorsed by the WHO General Assembly in May 1980.
However, even as the disease was being eradicated there still remained stocks of smallpox virus in many laboratories.
Accelerated by two cases of smallpox in 1978, one fatal (Janet Parker), caused by an accidental and unexplained containment breach at a laboratory at the University of Birmingham Medical School, the WHO ensured that known stocks of smallpox virus were either destroyed or moved to safer laboratories.
By 1979 only four laboratories were known to have smallpox virus.
All English stocks held at St Mary's Hospital, London were transferred to more secure facilities at Porton Down and then to the US at the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia in 1982, and all South African stocks were destroyed in 1983.
By 1984 the only known stocks were kept at the CDC in the US and the State Research Center of Virology and Biotechnology (VECTOR) in Koltsovo, Russia.
These states report that their repositories are for possible antibioweaponry research and insurance if some obscure reservoir of natural smallpox is discovered in the future. 
The last cases of smallpox in the world occurred in an outbreak of two cases (one of which was fatal) in Birmingham, UK in 1978.
A medical photographer, Janet Parker, contracted the disease at the University of Birmingham Medical School and died on September 11, 1978, after which the scientist responsible for smallpox research at the university committed suicide.
In light of this incident, all known stocks of smallpox were destroyed or transferred to one of two WHO reference laboratories which had BSL-4 facilities; the Centers for Disease Control and Prevention (CDC) in the United States and the State Research Center of Virology and Biotechnology VECTOR in Koltsovo, Russia.
In 1986, the World Health Organization first recommended destruction of the virus, and later set the date of destruction to be 30 December 1993.
This was postponed to 30 June 1999.
Due to resistance from the U.S. and Russia, in 2002 the World Health Assembly agreed to permit the temporary retention of the virus stocks for specific research purposes.
Destroying existing stocks would reduce the risk involved with ongoing smallpox research; the stocks are not needed to respond to a smallpox outbreak.
Some scientists have argued that the stocks may be useful in developing new vaccines, antiviral drugs, and diagnostic tests; however, a 2010 review by a team of public health experts appointed by the World Health Organization concluded that no essential public health purpose is served by the U.S. and Russia continuing to retain virus stocks.
The latter view is frequently supported in the scientific community, particularly among veterans of the WHO Smallpox Eradication Program.
In March 2004 smallpox scabs were found tucked inside an envelope in a book on Civil War medicine in Santa Fe, New Mexico.
The envelope was labeled as containing scabs from a vaccination and gave scientists at the Centers for Disease Control and Prevention an opportunity to study the history of smallpox vaccination in the U.S. 
The last cases of smallpox in the world occurred in an outbreak of two cases (one of which was fatal) in Birmingham, UK in 1978.
A medical photographer, Janet Parker, contracted the disease at the University of Birmingham Medical School and died on September 11, 1978, In light of this incident, all known stocks of smallpox were destroyed or transferred to one of two WHO reference laboratories which had BSL-4 facilities — the Centers for Disease Control and Prevention (CDC) in the United States and the State Research Center of Virology and Biotechnology VECTOR in Koltsovo, Russia.
In 1986, the WHO first recommended destruction of the virus, and later set the date of destruction to be 30 December 1993.
This was postponed to 30 June 1999.
Due to resistance from the U.S. and Russia, in 2002 the World Health Assembly agreed to permit the temporary retention of the virus stocks for specific research purposes.
Destroying existing stocks would reduce the risk involved with ongoing smallpox research; the stocks are not needed to respond to a smallpox outbreak.
Some scientists have argued that the stocks may be useful in developing new vaccines, antiviral drugs, and diagnostic tests; A 2010 review by a team of public health experts appointed by the WHO, however, concluded that no essential public health purpose is served by the U.S. and Russia continuing to retain virus stocks.
The latter view is frequently supported in the scientific community, particularly among veterans of the WHO Smallpox Eradication Program. 
The smallpox virus retention controversy is a debate that has been ongoing among international scientists and other officials since smallpox was declared eradicated by the World Health Organization (WHO) in 1979.
It centers on whether to finally and irreversibly destroy the two last remnants of the virus that causes smallpox, which reside in government laboratories in the United States and Russia.
Advocates of final destruction maintain that there is no longer any valid rationale for retaining the samples, which represent a hazard, while opponents of it maintain that the samples are needed for further research as smallpox virus may still exist in the world outside of the two labs, and thus may re-emerge, particularly as a bio-weapon.