Posted on October 2, 2017

The flu is worrying, particularly for doctors and parents with young children. The key question becomes, “Should I vaccinate this year?” followed closely by, “When should I vaccinate?” The answer to both questions often depends on another, “When does flu season 2017-2018 start?”

In past years, the effectiveness of flu vaccines has been reported at 43% to 73%. Effectiveness depends heavily on whether the World Health Organization (WHO) and the FDA select the right viral strain. Also, there’s no good consensus on how long a flu shot lasts. For these reasons, doctors and patients alike often want to know when flu season starts. As of 10/2/17, the article below presents the best-guess answer, based on flu season dates and severity in Asia and search data for “flu season” in Google.

With all this in mind, the medical community strongly encourages everyone to get the flu vaccination every year.

When Does Flu Season 2017-2018 Start?

Flu season 2017-2018 (also known as flu season 2018) has most likely already started. Cases have been reported around the U.S., though the season most likely won’t approach its highest numbers of cases until December or January. In America, flu season typically starts in September, takes hold as the air gets colder, and peaks in December or January. The number of new cases then declines through March or April.

Flu season 2017 started early for Australia. While that may mean an early flu season 2018 for the U.S., the flu can change severity between one region and the next. Online searches for “flu season” started earlier than normal and have already reached a higher level than most years. This fact may also point to an earlier start to flu season.

Flu Season 2017-2018 in the US: Clues from Asia

Flu season in the U.S. often follows flu season in Asia. That means we can look at Asia’s flu season for clues about what strains will hit the U.S. We can also use flu season data in China or Australia as a guide to when the season will hit here in the U.S. and how severe it’ll be.

However, take the data with several grains of salt. Australia often sees strong flu seasons when the U.S. has weak ones, and vice versa.

Flu Season 2017-2018 in Australia Predicts Strong US Season

Via: Immunisation Coalition
Articles in the New York Times, CNN Health, and other media outlets point to an extreme and early flu season in Australia for 2017-2018. This, they say, is a red flag that the U.S. may also see an earlier, more intense flu season. The data behind these articles does appear to bear out a high level of concern.

The articles cite an Australia Department of Health influenza report that mentions 2.5 times more influenza cases than the same time last year. However, the report doesn’t say flu season is 2.5 times worse this year, or that the cases are more severe than usual.

What the report does say:

  • There are 2.5 times more flu cases in Australia than at this time last year.
  • The increase may be caused in part by an earlier start to flu season.
  • At least part of the uptick may come from the introduction of quicker testing.
  • Cases of flu this year are less severe than in years past.
  • The peak season week in 2017-2018 has seen “comparable or higher” levels to previous years.

While the Department of Health report doesn’t expressly say this year’s flu season is twice as bad as normal, data from the Immunisation Coalition says exactly that.

The graph above from the Coalition shows more than double the confirmed cases this flu season than in the past five years. This does seem to herald bad news for the 2017-18 flu season in the U.S.

One big caveat: The severity of flu season in Australia doesn’t always correlate with severity in the U.S. In many cases, a strong flu season in Australia correlates to a weak one in the U.S. and vice versa.

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Flu Season 2017-2018 New Zealand

According to the World Health Organization (WHO) flu season in New Zealand is past peak for 2017-2018. The WHO reports a weaker than average intensity for the flu there compared to past years.

Flu Season 2017-2018 South Africa

South Africa’s flu season has peaked for the 2017-2018 season. Based on positive tests for influenza, the number of flu cases this season was moderate compared to the previous 10 seasons.

When Did Interest for Flu Season 2017-2018 Start?

The question “When does flu season 2018 start” or “When does the 2017-2018 flu season start” usually gets the answer, “There’s no way to tell for sure.” That’s because the flu is unpredictable.

That said, the number of online searches for “When does flu season start” may give some indication of the start of flu season itself. One possible reason? More people may search for information on flu season when they see people around them getting sick.

The graph below shows elevated search traffic for the term “flu season” in August and September of 2017 compared to previous years.

Did Flu Season Searches Start Earlier in 2017-2018?

Per the graph above, by late September of 2017, interest in the flu had already passed the peak level for the 2016-2017 flu season. In other words, search interest for flu season in September 2017 is already greater than interest in all of 2016.

This elevated interest in “flu season” for 2017-2018 doesn’t necessarily mean a stronger season. It may just mean more people are sick right now in general, and they’re wondering if they’ve caught the flu. Increased interest could also mean the flu is more prevalent in the media than in years past. In other words, while this year’s flu season may be no either less severe or more severe than others, flu season panic does definitely seem greatly elevated.

Finally, search traffic for “flu season 2017-2018” comes at least partially from Australia, which is seeing a stronger than normal flu season. That, plus media attention in the U.S., could very well be the source of the record September 2017 interest in influenza.

Did the 2018 Flu Season Start Earlier in the U.S.?

Aside from Australia, Asia, and search traffic data, is the U.S. seeing an earlier start to the 2018 flu season (and late 2017)? That remains to be seen, but looking at CDC data suggests this year’s flu season may start earlier than most.

Flu season got into full swing in December or January in seven of the past nine years. In one year, flu season started in November. In another, it started in September. That year (2009-2010) saw a very early flu season. Search interest in the flu also started in August, just like in this flu season (2017-2018). If search interest is any indication, we could indeed see an early start to the season in 2017-18.

As of 10/2/17, influenza does seem to be spreading in the United States. CDC data has yet to be released, but calls to several clinics in New York, Massachusetts, California, Oregon, Illinois, Missouri, and West Virginia confirm at least some cases of the flu.

Does a Weaker Flu Mean a Worse Flu Season?

Weaker intensity strains of influenza may lead to a higher number of cases. That’s because people with lower-intensity flu are less likely to stay home or otherwise isolate themselves from others.

If someone has a relatively mild case of the flu, she’s more likely to go to work or “tough it out.” That gives the virus more chance to spread to others. By contrast, if that same person is bedridden or “knocked on her back” by the flu, she’s a lot less likely to be out and about where the virus can infect others. By that reasoning, a virus that causes less intense symptoms can actually spread more effectively, causing more cases.

One paradoxical side effect of a weaker flu strain is that it may cause more deaths. That’s because it can infect more of the very young, very old, or immunocompromised population. This actually seems to have happened so far in flu season 2017 in Australia. The Department of Health reports less clinical severity there than in recent years, but CNN cites 52 deaths this year vs 27 by this time last year.

When Did Flu Season 2016-2017 Start in the U.S.?

Cases of the flu in the U.S. started appearing as early as September. That’s not out of the ordinary. Unfortunately, isolated cases don’t give a good idea of the size and scope of the epidemic.

Based on data from previous years, all we can really say about flu season dates for 2017-2018 is that it’ll really get going sometime between September and December. In fact, according to CDC data from the last ten years, there’s a 78% chance flu season will ramp up in earnest in December, and a 22% chance we’ll see a serious upsurge in cases much earlier, in October or November.

When Does Flu Season Usually Start?

Via: CDC
Flu season generally starts in September or October in the U.S., with cases ramping up in November/December and reaching a peak In December-March. The most common peak month for flu in the U.S. is February, as the CDC graph above shows.

Flu season typically starts 3-4 months before its peak. That means for the 2017-2018 season, cases of the disease that started to appear in September could herald a peak in December or January.

Flu Season Dates by Year

The table below shows when flu season started in the years between 2009 and 2017. The 2018 flu season likely started in September. It will likely peak in November or December.

The 2009 flu season’s September kickoff was earlier than any other in the U.S. in the last 10 years. That season was also the most severe on record, reaching pandemic status. Most other flu seasons since 2009 have started in December or January. The weakest was in 2011.

In the table, we’ve included the month when online searches for “flu season” picked up to 50% of their final levels.

When Does Flu Season Start?
Year Flu Search Start Flu Season Start
2009 August September
2010 September January
2011 August December
2012 December November
2013 September December
2014 September December
2015 September January
2016 October December
2017 January January
2018 August Sep-Dec?


What Months Are Flu Season?

In the U.S., flu season typically runs through fall, winter, and spring, with a peak in the winter. That means September through April are all possible flu season months.

A recent study may have unearthed a reason for the winter-flu connection. Flu viruses are less stable at warm temperatures in the 80’s. They transmit best at low humidity and high temperature. That makes the coldest winter months the most likely time to get the flu. In fact the word “influenza” means “influence,” and may refer to the “influence of cold weather.”

How Many People Die Each Year from Flu?

According to Harvard Health, 36,000 people die in the U.S. alone every year from flu. Over 200,000 are hospitalized because of the disease. In some years, as many as 56,000 Americans have died from influenza. Most of those are young children, elderly, or immunocompromised.

What Flu Vaccines Work in 2017-2018?

The first fact to know about vaccines in the 2017-2018 flu season is that the nasal spray flu vaccine (LAIV) isn’t recommended. The effectiveness of the nasally inhaled vaccine has been called into question. Therefore, the CDC recommends using only injectable vaccines this season.

According to the CDC, most vaccines this year protect against three strains of the flu:

  • An H1N1 strain
  • An H3N2 strain
  • A B/Victoria lineage strain

One vaccine adds protection from a B/Yamagata lineage strain.

Flu Vaccines and Egg Allergies

Flu vaccines have historically been made using chicken eggs. For the 2017-2018 flu season, a non-chicken-egg vaccine exists. This vaccine forms the H3N2 component of some immunization compounds. Though this is an exciting development, the CDC recommends people with severe egg allergies use caution when getting immunized.

For this season, people with mild egg allergies can get the flu shot. People with severe egg allergies should still get a shot, but should receive the vaccine in a medical setting where care can be offered if something goes wrong.

Will there Be a Flu Epidemic in 2017-2018?

There’s a flu epidemic in the U.S. each year, also known as “flu season.” In the 2009-2010 season there was a flu “pandemic,” which means “new disease that spreads worldwide.” The severity of that season was the worst of the past 10 years.

The devastating flu pandemic of 1918 infected approximately half a billion people. That’s about a third of the total global population at the time. It killed over 20 million people, including over 600,000 Americans. If those same percentages repeated today, the flu would kill 2.5 billion people worldwide and nearly two million in the U.S. That kind of devastation is unlikely, thanks to vaccines and antiviral drugs.

Key Takeaways

Flu season in the U.S. is unpredictable at best. An educated guess, plus a look at Australian data and U.S. search data, point to an earlier, more severe flu season in 2018. That said, the severity and timing of flu seasons in Asia in previous years haven’t traditionally forecast similar epidemics in the U.S. Further, search data may indicate nothing more than public interest fueled by media coverage of gruesome flu-related deaths.

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