According to the US Centers for Disease Control and Prevention (CDC), vaccination with seasonal flu vaccines made for the 2005 to 2009 seasons is unlikely to protect people against infection by the new A (H1N1) swine flu virus.

Scientists from the federal agency used banked blood samples taken before and after vaccination every year since 2005 and exposed them to the new swine flu virus. They reported the results of their investigations in the 19 May issue of their Morbidity and Mortality Weekly Report (MMWR).

The results showed that although samples from children had no antibodies, samples from adults vaccinated against seasonal flu showed a slight increase in antibodies against the novel influenza A (H1N1) virus but it was not possible to say whether this would be enough to protect them against infection. Because of this they concluded that a vaccine specific to the new strain was needed.

The result is consistent with the fact that the new A (H1N1) virus is genetically more different to the seasonal flu viruses that have circulated in the last few years than the seasonal flu viruses are from each other.

The new A (H1N1) virus shares only 72-73 per cent of the significant amino acid identity with the seasonal flu viruses, while the seasonal flu viruses shared 97 to 98 per cant among themselves.

The significant amino acids that the CDC investigators looked at are in the HA1 portion of the hemagglutinin molecule which is part of the surface of the virus that the immune system reacts to in order to make antibodies.

The strain of H1N1 that the CDC used in their tests was the A/California/04/09, and the strains of seasonal flu virus that they used were the A/New Caledonia/20/1999, A/Solomon Islands/3/2006 and A/Brisbane/59/2007.

Although the number of blood samples from children tested in their analysis was small, the CDC said the results showed that US children are largely “serologically naïve to the novel influenza A (H1N1) virus ” and that vaccination against seasonal flu (with either TIV, trivalent inactivated influenza vaccine or LAIV, live attenuated influenza vaccine) does not “not elicit any measurable level of cross-reactive antibody to the novel virus”.

Adults however, appear to have some degree of pre-existing immunity to the novel A (H1N1) virus strain, especially the over 60s.

The CDC suggested this could be because older adults have at some time in their lives been infected with a strain of seasonal flu that is genetically more closely related to the new H1N1 virus than the more recent ones have been.

The CDC investigators concluded that:

“Development of a strain-specific vaccine against the novel influenza A (H1N1) virus is needed for optimal protection against the virus among persons of all ages.”

Ongoing work to assess antibody response in different age groups may give more details later, they added.

The World Health Organization (WHO) estimates that the world capacity for producing vaccines against the new swine flu is about 2 billion doses. If people are likely to need two shots to get full immumity (this is still being debated), then that means there will only be enough vaccine for 15 out of every 100 people in the world.

Anne Schuchat, CDC’s deputy director for science and public health said earlier this week that she and her colleagues were working on the assumption that two doses will be needed, and they were keen to discover if groups in the population already have some pre-existing immunity to the virus, reported the Washington Post.

As of yesterday, 21 May at 06:00 GMT, the WHO reported that 41 countries have notified them of 11,034 officially confirmed cases of influenza A (H1N1) infection, including 85 deaths. All deaths have occurred in four countries: Mexico, USA, Canada and Costa Rica.

According to figures quoted by the Washington Post, the United States has counted 5,764 confirmed cases and nine deaths, in 47 states and the District of Columbia, but epidemiologists suggest the actual number of infections is over 100,000.

In the US nearly two-thirds of novel A (H1N1) infections are happening in people aged from 5 to 24 and less than 1 per cent are in people aged 65 and over, which is the reverse of what happens with seasonal flu. 40 per cent of cases serious enough to require hospitalization have been among 19 to 49 year olds.

“Serum Antibody Response to a Novel Influenza A (H1N1) Virus After Vaccination with Seasonal Influenza Vaccine .”
CDC, MMWR May 22, 2009 / Vol. 58 / No. 19.

Additional sources: Washington Post, WHO.

Written by: Catharine Paddock, PhD