According to the US Centers for Disease Control and Prevention, this year’s seasonal flu vaccine has only been 44 per cent effective. And depending on how you look at the effectiveness in the various strains, it would appear overall to be the least effective vaccine for since the 1997-98 flu season when the vaccine effectiveness was essentially zero, the CDC told a press conference.
The 44 per cent effectiveness figure comes from a CDC study whose findings are published in this week’s issue of the federal agency’s Morbidity and Mortality Weekly Report (MMWR), dated 17th April 2008.
The 44 per cent figure is the overall vaccine effectiveness for influenza type A (H3N2), and influenza type B. Type A H3N2 constitutes the majority of the strains circulating so far this year.
When broken down, the effectiveness of the trivalent inactivated vaccine (flu shot) in preventing medically attended laboratory confirmed influenza for type A (H3N2) was found to be 58 per cent, but for type B the effectiveness was found to be zero. So this year, the vaccine has essentially offered no protection against the type B strain.
The study was carried out at the CDC Marshfield clinic in Wisconsin, involving patients enrolled from January 21st to February 8th of 2008 who lived in and around Marshfield, Wisconsin.
Dr Dan Jernigan, Deputy Director, CDC Influenza Division, National Center for Immunization and Respiratory Diseases (NCIRD), briefed reporters in a web conference yesterday.
He said that most of the flu viruses circulating this year have been “less than optimally matched to the viruses in the vaccine”. He said the main strains circulating this year were type A H3N2, type A H1N1, and type B. He said that type A H3N2 constituted the majority of the strains circulating this year, and within those, 70 per cent were a series of strains called A-Brisbane10/ 2007.
Jernigan said that while the Brisbane strain had “drifted”, it was still “somewhat related” to A-Wisconsin strain, which is in this year’s vaccine. Of the type B, over 90 per cent are of the B Florida strain, which belongs to the Yamagada lineage, which is quite different to the Victoria lineage that is in this year’s flu vaccine.
The figure of 44 per cent means that the people in the study who received this year’s flu shot were 44 per cent less likely to have laboratory diagnosed influenza than those in the study who did not receive the flu shot. This figure is high enough to justify continuing to promote the public health message that people should be vaccinated, said Jernigan.
However, he pointed out that because of the less than optimal level of protection offered by the vaccine this year, health professionals and the public need to consider taking what he called the “three pronged approach” to protect against the flu this year.
The “three pronged approach” consists of first, get vaccinated, second take every day precautions such as covering your cough and washing your hands to prevent spreading of germs, and third, take anti-viral drugs, as recommended by your doctor.
Jernigan warned that if the B strains become dominant in the rest of this season, health care professionals should be prepared for an increased risk of vaccine failure and consider using anti-virals earlier to treat and prevent illness in people at higher risk of flu complications.
The Wisconsin study mentioned in the MMWR report was based on laboratory data, which is only part of the picture when assessing the effectiveness of the vaccine. Jernigan said the CDC was beginning to get early figures that showed “substantial cross protection against the predominant virus in the United States this season”, and this showed that “continuing to promote vaccination is beneficial even when some of the laboratory data might indicate a less than optimum match”.
A record number of Americans were vaccinated against the flu this year. Around 113 million doses of flu vaccine were delivered by drug companies in the US this year, more than ever before, and around 10 million more doses than last season, Dr Jeanne Santoli, Deputy Director, CDC Immunization Services Division, NCIRD told the news conference.
At the peak of the season this year, which was around the middle of February, flu deaths peaked to reach 9.1 per cent of all deaths in the US. This is similar to four years ago, when during the 2003-2004 season, flu deaths peaked at 10.4 per cent of all US deaths.
The flu epidemic is still ongoing in the US, with six states, Connecticut, Maine, Maryland, New York, Pennsylvania, and Vermont, still experiencing widespread infection.
The strains included in a seasonal flu vaccine are decided every year, when world experts get together to anticipate the strains of flu that are likely to circulate the globe in the coming flu season.
But, because of the lead times to produce the hundreds of millions of vaccine doses needed worldwide, it means the experts have to make the decision about the likely strains months before we know which strains will actually be circulating by the time the flu comes around. And during that time the risk is that things can go very differently. Flu viruses mutate and the balance among the strains changes.
So there are “good” years, when the vaccine strains match the circulating strains (these are when the match is about 70 per cent effective) and there are bad years, when the match can even be zero for some subtypes.
Depending on how you look at the vaccine’s effectiveness, for instance taking just the overall vaccine effectiveness figure, it appears that this year’s effectiveness is the lowest since the 1997-1998 season, when overall effectiveness was about 50 per cent. This emerged during a question and answer session between Jernigan and reporters.
“Interim Within-Season Estimate of the Effectiveness of Trivalent Inactivated Influenza Vaccine – Marshfield, Wisconsin, 2007-08 Influenza Season.”
Centers for Disease Control and Prevention.
Morbidity and Mortality Weekly Report (MMWR), 17 April 2008.
Source: CDC MMWR report, CDC web conference transcript (17 April).
Written by: Catharine Paddock, PhD