Other important differences relate to transmission, which is why it has been so hard to control COVID-19. The period of time that you are infectious appears to be longer with COVID-19 than with influenza. With a flu, people typically become contagious about a day before they have symptoms and remain contagious for about seven days after symptoms start. With COVID-19, our best estimate is that you can become infectious about two days before symptoms start, and remain contagious for up to 10 days afterward, although new information is coming out on this topic frequently.
There also appears to be a greater number of asymptomatic COVID-19 cases than asymptomatic flu cases. And the death rate for COVID-19 appears to be higher than the death rate for flu. There also appears to be more super-spreading events with COVID-19 than with flu: It seems to be transmitted more easily through the air, although both viruses are primarily spread by droplets. Finally, the risk of complications and death in healthy infants and children appears to be higher for flu than for COVID-19.
4. Is COVID-19 changing anything about this year’s flu vaccine?
The flu vaccine was developed this year in the same way it’s developed every year. The four strains of influenza that are going into this year’s flu shot were decided on in March, and the vaccine was distributed starting this month. I think it is unlikely that vaccinating for flu would cause any shortages or changes to the rollout of a COVID-19 vaccine.
Given concerns about an overburdened hospital system this fall and winter with admissions for both COVID-19 and influenza, an increased amount of flu vaccine is being made. The CDC reports that there will be 194 million to 198 million doses of flu vaccine available for this season, which is a record number. Unfortunately, only about 40% to 50% of adults in the United States get the vaccine, with rates commonly lower among low-income people and people of color — those most impacted by COVID-19.
5. Why do we need to get a flu shot every year?
The antibodies your immune system makes in response to the vaccine last only about six months. But also, influenza is a very tricky virus. Individual strains circulating in the community switch from year to year, so the vaccine from last year may not contain the strains that are circulating this year. In addition, individual strains of the virus constantly mutate, enabling them to evade our immune system and cause disease.
Stanford Health Care patients can contact their primary care physicians for information about scheduling a flu vaccination, which will be offered at clinics, as well as at drive-through and pop-up locations.