H1N1 Flu: More of Your Questions Answered

Guest blogger Matthew Hoffman, MD, has written for WebMD since 2006. He is a board-certified internist and is currently a fellow in pulmonary and critical care medicine at Emory University, where he also completed medical school and residency.

We received several good questions in response to my blog post last week, which I will answer here.

Mark asked: Does an episode of influenza confer immunity for subsequent episodes of influenza?

The answer is yes...and no. Sorry for the tricky reply, but influenza can be a tricky virus.

During an episode of influenza (or any viral infection), your body battles the virus and breaks it into fragments. White blood cells in your body carry these fragments around, "showing" them to other cells in your immune system. Those cells respond by making antibodies against that part of the virus. They also store a memory of that particular influenza virus fragment. Other immune cells do the same thing with other virus fragments - like pieces of a puzzle. If that virus ever shows up in your body again, the antibody response is almost immediate, and the bug is crushed before it can make you sick.

So yes - after an episode of influenza, you are immune to that particular influenza virus.

The catch is that there is no single "influenza virus." Rather, the influenza virus is an assembly of several modules that vary from virus to virus, and from year to year. These modules are called epitopes, but you can think of them as viral "spare parts." Two forms of the influenza virus might share many epitopes, or just a few. You are not immune to any form of influenza made of epitopes your immune system hasn't yet seen.

This part gets even trickier. The influenza virus recycles parts from previous years when it recreates itself each season. Previous influenza infection with a similar virus probably gives partial immunity to a new virus, if it's using many of the same spare parts as a flu virus you caught before. This may be why older Americans, who have been exposed to multiple forms of influenza over the years, seem less likely to catch H1N1 influenza, compared to young people.

H1N1 and seasonal influenza share a few, but not all of the same epitopes. They're enough unlike each other that your immune system won't recognize one after being infected with the other.

The bottom line: catching seasonal flu does not give immunity to H1N1 flu, and vice versa. And catching seasonal flu this year won't likely provide immunity against flu infection next year, because that virus will have reinvented itself with new epitopes.

Josh asked: Have there been any reports on lingering side effects after the flu or H1N1?

In the vast majority of people, influenza infection is a very uncomfortable, but brief illness. The worst symptoms start to improve in a few days, and most people feel much better within a week.

Other people take a while to get all their energy back after the flu. General weakness or fatigue, called postinfluenza asthenia, can last for several weeks in some people.

A small minority of people develop complications of influenza. These can include:

• Severe pneumonia (either from the flu virus, or a second infection from a bacteria).


• Myositis and rhabdomyolysis (muscle breakdown, causing pain and weakness)


• Extremely rarely, brain or spinal cord involvement

As you could guess, people experiencing these uncommon complications will have a longer course of illness, and a longer recovery from influenza.

Serious side effects from the seasonal flu vaccine are extremely rare. Most are allergic reactions, which occur rapidly and also resolve quickly if treated. In roughly one in one million people vaccinated, the influenza vaccine may cause Guillain-Barre syndrome, a nerve condition causing severe weakness (usually temporary).

The H1N1 vaccine is new, but it is made using the same materials and processes as the seasonal flu vaccine. Its side effects are expected to be mild, with an extremely low rate of serious side effects.

The Centers for Disease Control and Prevention and the Department of Health and Human Services have created a surveillance system to identify any unexpected side effects, which are unlikely.

John asked: My child has the H1N1, and is doing very well after taking Tamiflu. It's been 5 days, so when will he be in the clear? Does my son still need the H1N1 or any flu shot? Also, I had a mild sore throat and it's gone, did I or do I have the H1N1, and simply not experiencing the worst symptoms? What's the incubation period in the body? Can I still get sick later - next week?

Sounds like your son is having the typical course of H1N1 infection - a few days of feeling bad, and back on the mend within about 5 days. People with H1N1 are considered to be infectious from one day before until one week after their symptoms start, so he should be ready for school by the time you read this.

Remember, though, that other viral illnesses can mimic H1N1 influenza. Unless your son had an "RT-PCR test" proving he had H1N1, you don't know he had it for sure. (It's unlikely he did get the RT-PCR test; it's not used in most doctors' offices). He probably had a nasal swab test, which can be unreliable in diagnosing H1N1.

For these reasons, the CDC recommends that without an RT-PCR test proving H1N1 infection, everyone from 6 months to 24 years old should get the H1N1 vaccine, even if it seems like they had H1N1. The other groups who should get vaccinated are pregnant women, people caring for infants, health care workers, and people aged 25 to 64 with chronic medical conditions.

The CDC also recommends that everyone aged 6 months to 19 years should get the seasonal flu shot. You didn't specify his age, but it sounds like your son might be in this age group. Remember that even if his previous infection was H1N1, that doesn't provide immunity from seasonal flu, experts say. (And vice versa: seasonal flu infection doesn't create immunity against H1N1 influenza.)

As for your own symptoms, the incubation period for seasonal and H1N1 influenza is usually one to two days, but can take up to four days. If you're not sick by the time you're reading this, you probably won't be. It would be unusual to have only a mild sore throat from influenza, so you may have caught something else. That means you're still at risk for H1N1 and seasonal flu. Talk to your doctor about getting vaccinated, if you want to reduce your risk.