Flu shot when is it effective

Many other viruses, like RSV and rhinovirus, are coming back. And the flu likely will follow suit. While the US saw a steep rise in coronavirus infections in the winter, the spread of influenza was uncommonly light.

However, this year could return to the typical pattern, in which millions of people in the US catch the influenza virus, according to the Centers for Disease Control and Prevention , and tens of thousands die from flu-related causes.

With COVID vaccines and booster shots now authorized -- even mix-and-match shots -- intermingling is expected. And with that, a possible increase in the flu, alongside new waves of delta variant COVID infections at a time when peoples' stance on masking and social distancing have become political issues, not just public health concerns. We talked with three flu-vaccine experts about what to expect this flu season -- read on for their practical guidance and concerns.

For more on the flu, here's where you can get your shot in , which flu-shot side effects you may experience and why epidemiologists suggest taking both a COVID vaccine and flu shot. In the US -- and the rest of the northern hemisphere -- flu season usually runs October to May, according to the experts we spoke to.

This means that vaccination of study subjects is not randomized. This point estimate represents the reduction in risk provided by a flu vaccine. CDC vaccine effectiveness studies measure different outcomes. The confidence interval provides a lower boundary for the VE estimate e. Confidence intervals are important because they provide context for understanding the precision or exactness of a VE point estimate.

The wider the confidence interval, the less exact the point value estimate of vaccine effectiveness becomes. The width of a confidence interval is related in part to the number of participants in the study, and so studies that provide more precise estimates of VE and consequently, have a narrower confidence interval typically include a larger number of participants.

A report examining studies from the to the seasons concluded that the effectiveness of a flu vaccine may be influenced by vaccination the prior season or during many prior seasons 1 , 2. In some seasons, protection against influenza A H3N2 virus illness may have been lower for people vaccinated in the current season and the prior season compared with those who had only been vaccinated in the current season.

This fits with findings on immune response to vaccination that suggest repeated influenza vaccination can weaken the immune response to vaccination and especially to the H3N2 vaccine component. However, repeated annual vaccination also can be beneficial during some seasons, since sometimes people retain and carry over immune protection from one season to the next. During some seasons, people who missed getting vaccinated still had residual protection against influenza illness.

Information regarding flu vaccination history is particularly important to these types of evaluations, and can be difficult to confirm, as accurate vaccination records are not always readily available. People who choose to get vaccinated every year may have different characteristics and susceptibility to flu compared to those who do not seek vaccination every year.

CDC supports continued efforts to monitor the effects of repeat vaccination each year in part to understand the immune response to repeat vaccination better. However, based on the substantial burden of flu in the United States, and on the fact that most studies point to vaccination benefits, CDC recommends that yearly flu vaccination is the first and most important action people can take to protect against flu and its potentially serious complications.

Vaccine effectiveness studies that measure different outcomes are conducted to better understand the different kinds of benefits provided by vaccination. Ideally, public health researchers want to evaluate the benefits of vaccination against illness of varying severity. To do this, they assess how well flu vaccines work to prevent illness resulting in a doctor visit, or illness resulting in hospitalization, ICU admission, and even death associated with flu.

Because estimates of vaccine effectiveness may vary based on the outcome measured in addition to season, population studied, and other factors , results should be compared between studies that used the same outcome for estimating vaccine effectiveness.

Scientists continue to work on better ways to design, conduct and evaluate non-randomized i. CDC has been working with researchers at universities and hospitals since the flu season to estimate how well flu vaccine works through observational studies using laboratory-confirmed flu as the outcome.

These studies currently use a very accurate and sensitive laboratory test known as real-time RT-PCR reverse transcription polymerase chain reaction to confirm medically attended flu virus infections as a specific outcome.

Similar studies are being done in Australia, Canada and Europe. HAIVEN looked at how well flu vaccines protect against flu-related hospitalization among adults aged 18 and older.

IVY consists of 21 large, adult hospitals in 20 U. During the flu season, the network will also enroll patients hospitalized with flu. NVSN collects vaccine effectiveness data on pediatric hospitalizations with laboratory confirmed flu in children 18 years of age and younger.

The network was established in and includes the following eight U. The large numbers of flu-associated illnesses and deaths in the United States, combined with the evidence from many studies that show flu vaccines help to protect against flu illness and its potentially serious complications, support the current U.

CDC recommends a yearly flu vaccine as the first and most important step in protecting against flu viruses. An annual seasonal flu vaccine is the best way to help protect against flu. Vaccination has been shown to have many benefits including reducing the risk of flu illnesses, hospitalizations and even the risk of flu-related death in children.

While some people who get a flu vaccine may still get sick, flu vaccination has been shown in several studies to reduce severity of illness.

Flu vaccines cause antibodies to develop in the body about two weeks after vaccination. These antibodies provide protection against infection with circulating influenza viruses. Seasonal flu vaccines are designed to protect against the influenza viruses that research indicates will be most common during the upcoming season.

CDC recommends use of any licensed, age-appropriate influenza vaccine during the influenza season. No preference is expressed for any influenza vaccine over another. There are many vaccine options to choose from, but the most important thing is for all people 6 months and older to get a flu vaccine every year. If you have questions about which vaccine is best for you, talk to your doctor or other health care professional.

Everyone 6 months of age and older should get an influenza flu vaccine every season with rare exception. Vaccination to prevent flu and its potentially serious complications is particularly important for people who are at higher risk of developing serious flu complications. See People at Higher Risk of Developing Flu-Related Complications for a full list of age and health factors that confer increased risk. More information is available at Who Needs a Flu Vaccine.

Different influenza flu vaccines are approved for use in people in different age groups. In addition, some vaccines are not recommended for certain groups of people. September and October are generally good times to be vaccinated against flu.

Ideally, everyone should be vaccinated by the end of October. However, even if you are not able to get vaccinated until November or later, vaccination is still recommended because flu most commonly peaks in February and significant activity can continue into May. Find Vaccines. A flu vaccine is needed every year for two reasons. Second, because flu viruses are constantly changing, the composition of flu vaccines is reviewed annually, and vaccines are updated to protect against the viruses that research indicates will be most common during the upcoming flu season.

For the best protection, everyone 6 months and older should get vaccinated annually. It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection. Influenza flu vaccine effectiveness VE can vary. During years when the flu vaccine match is good, it is possible to measure substantial benefits from flu vaccination in terms of preventing flu illness and complications.

However, the benefits of flu vaccination will still vary, depending on characteristics of the person being vaccinated for example, their health and age , what influenza viruses are circulating that season and, potentially, which type of flu vaccine was used.

For information specific to this season, visit About the Current Flu Season. This is possible for the following reasons:. Some people who get vaccinated may still get sick with flu. However, flu vaccination has been shown in some studies to reduce severity of illness in people who get vaccinated but still get sick.

A study showed that flu vaccination reduced deaths, intensive care unit ICU admissions, ICU length of stay, and overall duration of hospitalization among hospitalized adults with flu. There are many reasons to get an influenza flu vaccine each year.

This is because some flu vaccines are made using eggs. If you're ill with a high temperature, it's best to wait until you're better before having the flu vaccine.

Flu vaccines help protect against the main types of flu viruses, although there's still a chance you might get flu. If you do get flu after vaccination, it's likely to be milder and not last as long. Having the flu vaccine will also stop you spreading flu to other people who may be more at risk of serious problems from flu.

Flu vaccines are very safe. All adult flu vaccines are given by injection into the muscle of the upper arm. It's very rare for anyone to have a serious allergic reaction anaphylaxis to the flu vaccine. If this does happen, it usually happens within minutes. The person who vaccinates you will be trained to deal with allergic reactions and treat them immediately. Anyone can report a suspected side effect of a vaccine through the Yellow Card Scheme.

There are several types of injected flu vaccine. None of them contain live viruses so they cannot give you flu.