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The scientists are testing the vaccine as a regular shot and as an intranasal spray with the idea of stopping the virus in the respiratory tract before it causes a broad infection.

A switch from glutamine to leucinein resulting from just one mutation could switch the virus from being able to easily bind to avian, or bird, receptors to favouring human receptors in the respiratory tract.

Pope Francis died Monday in Rome at the age of 88, just weeks after he was released from hospital following a respiratory tract infection that developed into pneumonia.

In contrast, those that had been exposed to the seasonal flu beforehand had virus only in the respiratory tract — and at pretty low levels.

In an update on Monday, the Vatican said the pontiff has a "polymicrobial infection" of his respiratory tract, which has required a change in his treatment.