Blood pressure (BP) is the force exerted by circulating blood against the walls of the arteries. Maintaining healthy blood pressure is important for preventing serious long-term health issues. Many people managing cardiovascular health are concerned about whether the influenza vaccine can affect this measurement. Investigating the direct effect of the flu shot helps clarify the vaccine’s safety profile for the general public.
Flu Shot and Blood Pressure Fluctuation
The direct answer is that the flu shot does not typically cause a long-term or clinically concerning increase in blood pressure. Research indicates the vaccine does not lead to sustained hypertension or a lasting elevation in baseline blood pressure. For people already managing high blood pressure, the vaccine is considered safe and does not pose a significant risk to their control regimen.
A very minor and temporary elevation, or transient spike, can occur immediately following vaccination. This brief increase usually lasts only a few hours to a day and is not considered a sustained side effect. This short-lived fluctuation is not the same as developing true hypertension, which involves consistently high readings over an extended period.
Understanding Temporary Immune Response
The short-term blood pressure elevation is typically attributed to one of two physiological mechanisms.
Immune Response
The first mechanism is the body’s normal, self-limited immune response to the vaccine, which mimics the early stages of responding to an actual infection. The vaccine introduces harmless viral components, prompting the immune system to activate its defenses. This activation involves releasing signaling molecules, such as cytokines, which are part of the inflammatory response. These molecules can transiently affect the dilation and constriction of blood vessels, contributing to a slight and temporary rise in pressure.
Anxiety and Stress
The second common cause is an anxiety-related response, often referred to as a vasovagal reaction or “white coat syndrome.” For people anxious about needles or medical procedures, the stress of the vaccination can trigger a temporary spike in heart rate and blood pressure. This stress-induced response is neurological, not immunological, and resolves quickly once the anxiety subsides. This minor fluctuation is a normal bodily reaction to stress, not an adverse vaccine reaction.
Cardiovascular Risks of Influenza Infection
The minimal, transient nature of blood pressure changes from the vaccine stands in sharp contrast to the severe cardiovascular risks associated with contracting the actual influenza virus. A full influenza infection causes widespread systemic inflammation, placing immense stress on the heart and vascular system.
In the month following a confirmed influenza infection, the risk of having a heart attack is approximately four times higher than the baseline risk. Similarly, the risk of having a stroke can increase by as much as five times during the same period. The inflammation triggered by the virus can destabilize existing fatty plaques within the arteries, making them more likely to rupture and form a clot.
This systemic stress can also lead to myocarditis (inflammation of the heart muscle) or pericarditis (inflammation of the sac surrounding the heart). These complications can severely impair the heart’s ability to function properly. Conversely, vaccination offers a protective benefit; one study found it associated with a 16% reduced risk of death from cardiovascular causes in patients with hypertension.