similarity to H2N2 but also its antigenic distinction. The H3N2 pandemic flu strain contained genes from a low-pathogenicity avian influenza virus. Specifically, it had acquired a new hemagglutinin gene and a new PB1 gene, while it preserved the neuraminidase and five other genes from the preexisting human H2N2 strain. The new hemagglutinin helped H3N2 evade preexisting immunity in humans. It is possible that the new PB1 facilitated viral replication and human-to-human transmission. The new subtype arose in pigs coinfected with avian and human viruses and was soon transferred to humans. Swine were considered the original "intermediate host" for influenza because they supported reassortment of divergent subtypes. However, other hosts appear capable of similar coinfection (such as many poultry species), and direct transmission of avian viruses to humans is possible. H1N1, associated with the 1918 flu pandemic, may have been transmitted directly from birds to humans. Accumulated antibodies to the neuraminidase or internal proteins may have resulted in many fewer casualties than most other pandemics. However, cross-immunity within and between subtypes of influenza is poorly understood. The basic reproduction number of the flu in this period was estimated at 1.80. Mortality The estimates of the total death toll due to Hong Kong flu (from its beginning in July 1968 until the outbreak faded during the winter of 1969–70) vary: The World Health Organization and Encyclopaedia Britannica estimated the number of deaths due to Hong Kong flu to be between 1 and 4 million globally. The United States Centers for Disease Control and Prevention (CDC) estimated that, in total, the virus caused the deaths of 1 million people worldwide. However, the death rate from the Hong Kong flu was lower than most other 20th-century pandemics. The World Health Organization estimated the case fatality rate of Hong Kong flu to be lower than 0.2%. The disease was allowed to spread through the population without restrictions on economic activity, and a vaccine created by American microbiologist Maurice Hilleman and his team became available four months after it had started. Fewer people died during this pandemic than in previous pandemics for several reasons: Some immunity against the N2 flu virus may have been retained in populations struck by the Asian Flu strains that had been circulating since 1957. The pandemic did not gain momentum until near the winter school holidays in the Northern Hemisphere, thus limiting the infection's spread. Improved medical care gave vital support