The new virus is considered a swine flu because one of the two main proteins on its surface is similar to proteins on pig viruses, though its genome contains genes from pig, bird and human viruses (Image: proman/Stock.Xchng)

Hundreds people in Mexico and an increasing number in other countries have come down with a new kind of swine flu. People are concerned because some of those infected in Mexico have died, and because this is the kind of virus that could become a serious worldwide epidemic (see Deadly new flu virus in US and Mexico may go pandemic and Threat level for flu pandemic raised).

Should I worry about this flu?

That depends on two things: how severe this flu is, and how far it spreads. Its severity is still unknown. Those who died in Mexico were young adults who don’t often die of flu, so we know this virus can be serious. But it isn’t always this bad: the cases picked up in the US were mild. Outbreak investigators are now trying to find out how many people have had the virus, and how many of those were seriously ill, to get an idea of how bad it is.


Will it spread to where I live?

That depends, again, on two different things: whether the virus is transported to your region, and how efficiently it spreads between people.

So many people travel globally now that, as long as this virus keeps infecting people, it is unlikely not to get to where you live. Some countries are already using infrared cameras to spot people with fevers on flights from affected areas. But that won’t stop it entirely, since five days can pass before an infected person shows symptoms, and the virus can spread before symptoms start.

The big question is how efficiently it spreads once it lands. From the number of cases in Mexico and the fact that those infected in the US had not been in contact with pigs or each other, we know that it can spread from human to human, and has done so for weeks at least. Investigators are conducting tests to see whether people who contacted known cases have also been infected to try to assess how easily it spreads. Preliminary observations in the US suggest it has spread readily to contacts of known cases.

Similar swine flu viruses have jumped from pigs to people before and have always petered out without causing a pandemic because they were not good enough at spreading in people. This virus may do the same thing.

Does this virus mean I shouldn’t eat pork?

No. This virus is named swine flu because one of its surface proteins is most similar to viruses that usually infect pigs, and the whole thing is of a type that has been spreading in North American pigs for years. But this particular virus is spreading in people and we don’t yet know if it infects pigs. In any case, cooking kills the virus. Wash your hands after handling meat.

Can I travel to other countries?

Yes. There are no official travel advisories against going to affected regions of Mexico, but the cases of this virus being discovered in other countries are all in people who had recently travelled there.

27 April UPDATE: At a European Commission briefing, the EU health commissioner Andorra Vassiliou asked Europeans to postpone nonessential travel to the United States or Mexico.

Europeans “should avoid travelling to Mexico or the United States of America unless it is very urgent for them,” she said.

The US Centers for Disease Control has also warned against all non-essential travel to Mexico. The World Health Organisation however has called on countries not to close their borders, as epidemic models have shown this will have little effect on the spread of a pandemic.

What if it causes a pandemic?

Most countries in the world have pandemic plans, on paper at least. They can respond with vaccines, drugs, and measures called “social distancing”, aimed at limiting human contacts that spread flu. Mexico has already done this, by banning public gatherings and closing schools in affected areas. Modelling suggests this can be effective.

The Mexican swine flu virus is susceptible to the most widely stockpiled flu antiviral drugs, Tamiflu and its relatives. But viruses of the same family can readily develop resistance to these drugs, so no one knows how long the drugs will remain effective.

There are no stocks of vaccine to this flu. The US has already created a “seed strain” from it, a virus that can be grown to make vaccines. Because of fears that H5N1 bird flu would go pandemic, vaccine companies and regulators, especially in Europe, have developed procedures over the past few years for rapidly approving and manufacturing pandemic vaccines in factories that normally make regular human flu vaccines.

The question now is whether and when they will switch production to a pandemic vaccine. If they do, the question will be how many doses they can produce, and how fast. Researchers are trying to find ways to stretch vaccine stocks, but there is no commonly agreed approach yet.

They won’t be able to make much vaccine for several months. But if there are several waves, each several months apart, as there have been in past pandemics, they might have time to make stocks of the vaccine before the next wave – if there is one.

Why are we worried this virus could go pandemic?

Because it is new. Flu constantly evolves, mostly with small changes to its surface proteins. There are usually enough differences between this year’s flu and whatever you had last time to allow the new virus to evade your immune defenses and cause an infection, but its similarities to your last case mean you still have partial immunity to it.

But flu surface proteins come in 16 different families, and viruses interbreed and swap genes. Occasionally a new flu virus that has picked up completely new surface proteins from pig or bird flu viruses circulates in people. Since no humans have been exposed to it before, no one has any immunity. Those viruses cause pandemics.

The Mexican swine flu is like this. It is of the H1N1 family, named for its two main surface proteins. There are other, mild H1N1 viruses circulating in humans as ordinary flu. But pig H1’s are somewhat different, and according to the CDC, immunity provided by the current H1N1 vaccine for humans does not protect against the Mexican H1.

The bad 1918 pandemic was an H1N1 virus, and there is evidence that people exposed to earlier human H1 flu had some immunity to it. The 1918 virus was different from this one, however, because its H surface protein was from birds.

What should I do to prepare for a pandemic?

We don’t know yet if this will be one – but quiet preparation can’t hurt. Cover coughs and sneezes and wash your hands a lot. Don’t run down your ability to fight infection – eat well and be sensible. Some scientists recommend going on statins as there is some evidence they reduce death rates from flu in people who tend to die from it in normal years.