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Time to Get a Flu Shot


By Christina Johnson   |   August 31, 2017

​San Diego may enjoy a year-round mild climate, but that doesn’t mean we escape flu season. In fact, people living in the tropics get the flu at similar rates as elsewhere; it’s just spread out over the year. Our fair weather is no excuse not to roll up a shirt sleeve and get a flu shot.

An annual flu vaccine is the best hedge against contracting influenza or spreading the flu virus to others, says the Centers for Disease Control (CDC). Among those most vulnerable to serious infection are infants, older adults and the sick.

“The flu shot is not completely effective, but it reduces the probability and severity of infection and the probability of transmitting the virus to others,” said Douglas Richman, MD, an infectious disease specialist and Distinguished Professor in the Department of Pathology at UC San Diego School of Medicine. “The vaccine is completely safe and the consequences of influenza can be very serious. The only negative is that it is not 100 percent effective, but it’s the best we have.”

flu shot

The CDC estimates that there have been 140,000 to 710,000 flu-related hospitalizations and 12,000 to 56,000 flu-related deaths since 2010.

Why isn’t the flu shot 100 percent effective?

The flu vaccine contains three or four strains of the flu virus predicted to be the most common in each season. The selection of which strains to include in the vaccine is based on currently circulating viruses, notably the viral strains that made people in the southern hemisphere sick during its flu season (from June to September).

“Usually, one of the strains circulating from the southern to the northern hemisphere will become common during our flu season,” Richman said. “But viruses are smarter than we are.”

The influenza virus is always mutating and the prediction may be wrong. However, even when the match is not perfect, the flu shot confers benefits. The antibodies your body produces in response to the initial vaccine will reduce the severity of the flu, Richman said.

“Designing a vaccine that will be more universal in its effectiveness is a major area of research,” said Richman.

Why not let the body fight infections naturally?

The flu vaccine contains the same viruses that are in the environment naturally, only in an inactivated (killed) form. This means your body reacts to the viruses as if they were alive and virulent, but without any risk of actually getting influenza. “Vaccines do not weaken your immune system or rob your body of the opportunity to get stronger on its own, just the opposite," said Richman. "They strengthen your body’s natural immune function.”

If I’m young and healthy, why get a flu shot?

Young, healthy individuals can also get serious flu infections that may lead to hospitalization, and the residual effect of even a non-serious flu infection can cause impaired lung function for six weeks, according to Richman. Another concern is that you will spread the virus to others who may not have the strength to fight off infection. “This is why if you work in the medical profession, we think it is unethical to forego a flu shot,” Richman said.

What viruses will the 2017-18 flu vaccines protect against?

Flu shots can contain three strains of the influenza virus (a trivalent vaccine) or four strains (a quadrivalent vaccine).

Based on circulating viral strains, this season's flu vaccine is recommended to contain:

  • A/Michigan/45/2015 (H1N1)pdm09-like virus
  • A/Hong Kong/4801/2014 (H3N2)-like virus
  • B/Brisbane/60/2008-like (B/Victoria lineage) virus, and
  • B/Phuket/3073/2013-like (B/Yamagata lineage) virus (in the quadrivalent vaccine only)

The dose is tailored to pediatric patients (older than 6 months), adults and seniors, said Danielle Kulischak, PharmD, a pharmacist and pharmacy purchasing manager at UC San Diego Health.

Should I worry about the mercury in influenza vaccines?

UC San Diego Health uses a preservative-free influenza vaccine that does not contain any mercury, Kulischak said.

In addition, the mercury-containing preservative thimerosal that is added to some types of vaccines is not the same as the mercury in seafood. Thimerosal contains ethylmercury, which is cleared from the body much more quickly than methylmercuy, the form in seafood. Learn more about thimerosal from the CDC.

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