After a slow start, the flu season has taken off.
“There was a substantial uptick between Christmas and New Year’s,” said William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center in Nashville, Tenn. “We don’t anticipate last year’s doozy of a year will be repeated, but there is plenty of illness out there.”
In a mild year, influenza, a highly contagious viral infection of the respiratory system, kills as many as 12,000 people in the U.S., and in a bad year, it could be as many as 56,000.
As you try to avoid catching the bug, here are a few numbers to keep in mind:
Three to 6 feet: The distance at which flu may spread from one person to another. “That’s the breathing zone,” Dr. Schaffner said. “When I breathe out, if I’m infected, what you inhale contains the flu virus.”
24 to 48 hours: The length of time you may be sick without knowing it. When people fall ill, they tend to sequester themselves, but the wily flu virus delays sickening its carriers. “If I get infected, I can spread the flu for 24 to 48 hours before I get sick,” Dr. Schaffner said. “That’s in the virus’s favor. It makes it easier for it to spread.”
Four: Each year, the World Health Organization recommends four strains of flu virus to include in the vaccine, including two A strains and two B strains. (B viruses typically infect only humans, while A viruses infect both people and animals, contributing to their ability to mutate and spread.) Trivalent vaccines include both “A” strains and one “B” strain. Quadrivalent vaccines, which are becoming the norm, include all four.
10 days to two weeks: How long it takes for the vaccine to take effect. “It’s not like flipping a switch,” Dr. Schaffner said. “Your body has to recognize it has been inoculated with a vaccine, and it has to have time to build up antibodies.”
60%…or less: In a good year, people who get vaccinated against the flu are 60% less likely to get sick, but the level of protection may be much lower.
Flu viruses readily mutate, so the strains contained in vaccines may not precisely match circulating viruses, reducing the effectiveness of the immunization. The vaccines in the last two flu seasons were only 40% effective, and the 2014-15 vaccine was only 19% effective.
Flu cases spiked across the U.S. in December.
Recent Influenza levels by state
Weekly influenza hospitalization by age group
hospitalizations per 100,000 people
Note: Chart data ends Dec. 29.
Source: Centers for Disease Control and Prevention
Still, Dr. Schaffner cautioned that people undervalue the partial protection vaccines offer.
“Even if you get the flu after having had the vaccine,” he said, “you are substantially protected against the complications, pneumonia or death.”
One-and-a-half: A virus’s reproduction number is an estimate of its transmissibility, or how far and quickly it will spread. A reproduction number of 1.0 means each person infected with the virus will, on average, infect one other person.
In the 2009 pandemic, the H1N1 virus—the same virus that is dominant in most areas of the country this year—had an effective reproduction number of 1.4 to 1.5.
The reproduction number for the 1918 pandemic, which caused an estimated 675,000 deaths in the U.S., was around 1.8.
Compared with measles, which is often said to have a basic reproduction number of 12 to 18 in a population in which no one is immune, spreading the flu to one or two other people may not sound like a big deal.
But recall the legend of the rice and the chessboard: In one version, a king captivated by the game of chess told the sage who invented it to name his reward. The sage asked for one grain of rice for the first square of the chess board, two for the second, four for the third and so on, consecutively doubling the figure for each of the board’s 64 squares.
The ruler readily agreed to what sounded like a modest request. But the reward added up to nearly 18.5 quintillion grains of rice, or something like 318 billion tons.
When H1N1 swept the globe in 2009, 74 countries and territories reported laboratory-confirmed infections by June that year, according to the World Health Organization, and by one estimate, 24% of the population was eventually infected.
The strange benefit is that, having torn across the globe, the pandemic H1N1 is now somewhat less of a threat.
“It’s well-represented, whether you’ve been exposed to the wild virus or to the vaccine,” Dr. Schaffner said. “We may have, relatively speaking, a pretty well-protected population.”
But the experts aren’t ready to relax.
“Flu is fickle,” Dr. Schaffner said. “This could change next week.”
Write to Jo Craven McGinty at Jo.McGinty@wsj.com