Norwegian scientists today reported a pandemic H1N1 virus mutation that appears to be associated with severe disease, but a leading US flu expert said global data on the mutation don´t show a clear connection with severe illness.
A team from the Norwegian Institute of Public Health in Oslo reports that it found the mutation in 11 of 61 severe illness cases that were analyzed between July and December 2009. The mutation was not found in any of 205 mild cases that were analyzed between May 2009 and January 2010.
"This difference is statistically significant and our data are consistent with a possible relationship between this mutation and the clinical outcome," says the report by A. Kilander and colleagues, published today in Eurosurveillance. "To our knowledge, this is the first identification of a change in the pandemic virus that correlates with a severe clinical outcome."
However, Dr. Nancy Cox, director of the Influenza Division at the US Centers for Disease Control and Prevention (CDC), said global H1N1 data so far do not show a clear association between the mutation and severe illness.
"If you look globally you can see that this mutation is neither necessary nor sufficient for a severe or fatal outcome," Cox told CIDRAP News.
The mutation the Norwegians found in the severe cases is called D222G (a change from aspartic acid to glycine in position 222 in the HA1 subunit of the hemagglutinin protein), according to the report. The team found it by sequencing viruses from the 61 severe cases and 205 mild cases. D222G mutant viruses were detected in severe cases throughout the sampling period of July to early December.
Because the mutation is found in the receptor binding site of hemagglutinin, it may influence which types of cells and which parts of the respiratory tract the virus binds to, the report says.
"Our observations are consistent with an epidemiological pattern where the D222G substitution is absent or infrequent in circulating viruses, with the mutation arising sporadically in single cases where it may have contributed to severity of infection," the scientists write.
They suggest that the mutation may increase the virus´s ability to bind to lung cells, leading to more severe illness. But it is also possible, they say, that the mutation is more likely to occur in patients who fail to fend off the virus quickly and end up with the infection in their lungs.
A "large proportion" of the patients with severe cases had underlying health conditions that increased their risk, but some of the patients were previously healthy and had an initially mild illness that later became much more severe, the report says.
"We consider it likely that there is a causal relationship between the occurrence of the D222G mutation in this virus and severe disease," it states. But it adds that the mutation is clearly not required for a severe outcome, since most of the severe and fatal cases did not feature it.
Cox, however, said it´s too soon to conclude there is a causal relationship. She pointed to possible holes in the Norwegian analysis, along with global surveillance data and experimental data.
"They haven´t adjusted for a whole variety of confounders that could influence the appearance of the mutation, such as time between onset of illness and collection of specimens, which might differ in mild and severe cases," she said. Another possible confounder, she added, is the fact that some of the patients with severe illness had underlying conditions, which increase the risk of severe flu.
Some countries have reported the D222G mutation only in mild cases, while others have seen it as statistically linked with severe illness, but the latter didn´t rule out possible confounders, Cox said. She reported that the CDC has found the mutation in a total of eight cases, of which five were nonfatal. Some of the nonfatal cases were mild.
Cox also said a lab experiment in ferrets did not support a causal link with severe disease. Virologist Dr. Richard Webby infected ferrets with a wild-type H1N1 virus or with a strain virus engineered to contain the D222G mutation, and found that those infected with the mutant strain had no worse disease than the others, she said.
"So, so far what´s been done in animal models doesn´t reflect the association of severity with this mutation," Cox said.
Referring to the Norwegian report, she commented, "It´s intriguing, and there could be some association, but it´s certainly not as clear as they´ve painted it."
"I think additional work needs to be done before we conclude that there is a causal relationship," she said.
Kiland A, Rykkvin R, Dudman SG, et al. Observed association between the HA1 mutation D222G in the 2009 pandemic influenza A(H1N1) virus and severe clinical outcome, Norway 2009-2010. Eurosurveillnace 2010 Mar 4;15(9):2 [Full text]
Nov 20, 2009, WHO statement on previous report of mutation seen in severe cases in Norway