New Delhi: With almost no fresh Severe Acute Respiratory Syndrome (SARS) suspects being reported in the country, World Health Organisation (WHO) has said that infection in India may have been "mild", possibly due to presence of other similar viruses in the environment or mutation in the dreaded virus.

"The fact that 21 people tested positive means these people were infected with the SARS virus," health officials said.

However, these did not show any symptoms or exhibited only mild symptoms, they added.

"…it is possible that these are cases of mild infection… According to one hypothesis, this points to the possibility of cross immunity existing between the new corona virus with the already circulating corona virus that causes common cold in human beings and diseases in animals such as pigs, birds and chicken," Dr N Kumara Rai from the WHO said.

For example, the vaccine to prevent smallpox is manufactured using live virus that causes cowpox in cow. The immunity to cowpox virus also confers immunity against the smallpox virus, he explained.

Another possible reason could be that India got infection by a less virulent mutated virus. But, it means that the virus may also become more virulent later, he said.

"Viral immunity is a complex thing. Usually (like chicken pox, measles, mumps), it is long lasting and the second episode does not occur. However, if the virus mutates or changes its antigenic characteristics, it can fool the body's immune system," Prof Anoop Misra from the AIIMS said.

PTI