A November 2009 New England Journal of Medicine article recommended that flu patients whose chest X-ray indicates pneumonia receive both antivirals and antibiotics.

[25] The WHO collaborated with the US Centers for Disease Control and Prevention (USCDC) and Netherlands Institute for Health Services Research (NIVEL) to produce two independent estimates of the influenza deaths that occurred during the global pandemic using two distinct methodologies.

[41] In 2009, U.S. agricultural officials speculated, although emphasizing that there was no way to prove their hypothesis, that "contrary to the popular assumption that the new swine flu pandemic arose on factory farms in Mexico, [the virus] most likely emerged in pigs in Asia, but then traveled to North America in a human.

[56] According to WHO statistics (as of July 2010), the virus had killed more than 18,000 people since it appeared in April 2009; however, they state that the total mortality (including deaths unconfirmed or unreported) from the H1N1 strain is "unquestionably higher".

[62] However, by 2012, research showed that as many as 579,000 people could have been killed by the disease, as only those fatalities confirmed by laboratory testing were included in the original number, and meant that many without access to health facilities went uncounted.

[71] The World Health Organization reports that the clinical picture in severe cases is strikingly different from the disease pattern seen during epidemics of seasonal influenza.

[73] A United Kingdom investigation of risk factors for hospitalisation and poor outcome with pandemic A/H1N1 influenza looked at 631 patients from 55 hospitals admitted with confirmed infection from May through September 2009.

[76] An article published in JAMA in September 2010[77] challenged previous reports and stated that children infected in the 2009 flu pandemic were no more likely to be hospitalised with complications or get pneumonia than those who catch seasonal strains.

[85][86] While it has been thought that these findings suggest the partial immunity in older adults may be due to previous exposure to similar seasonal influenza viruses, a November 2009 study of a rural unvaccinated population in China found only a 0.3% cross-reactive antibody reaction to the H1N1 strain, suggesting that previous vaccinations for seasonal flu and not exposure may have resulted in the immunity found in the older U.S.

[103][104] Health officials worldwide were also concerned because the virus was new and could easily mutate and become more virulent, even though most flu symptoms were mild and lasted only a few days without treatment.

Officials also urged communities, businesses, and individuals to make contingency plans for possible school closures, multiple employee absences for illness, surges of patients in hospitals, and other effects of potentially widespread outbreaks.

[106][107] In February 2010, the CDC's Advisory Committee on Immunization Practices voted for "universal" flu vaccination in the U.S. to include all people over six months of age.

[109] On 6 May 2009, the Public Health Agency of Canada announced that their National Microbiology Laboratory (NML) had mapped the genetic code of the swine flu virus, the first time that had been done.

[114] Preparations included planning a second influenza vaccination program in addition to the one for seasonal flu, and improving coordination between federal, state, and local governments and private health providers.

[114] On 24 October 2009, U.S. President Obama declared swine flu a national emergency, giving Secretary of Health and Human Services Kathleen Sebelius authority to grant waivers to requesting hospitals from usual federal requirements.

[120] In March 2010, the Council of Europe launched an enquiry into "the influence of the pharmaceutical companies on the global swine flu campaign", and a preliminary report was in preparation.

[121] On 12 April 2010, Keiji Fukuda, the WHO's top influenza expert, stated that the system leading to the declaration of a pandemic led to confusion about H1N1 circulating around the world and he expressed concern that there was a failure to communicate in regard to uncertainties about the new virus, which turned out to be not as deadly as feared.

"[122] In June 2010, Fiona Godlee, editor-in-chief of the BMJ, published an editorial which criticised the WHO, saying that an investigation had disclosed that some of the experts advising WHO on the pandemic had financial ties with drug companies which were producing antivirals and vaccines.

[123] Margaret Chan, Director-General of the WHO, replied stating, "Without question, the BMJ feature and editorial will leave many readers with the impression that WHO's decision to declare a pandemic was at least partially influenced by a desire to boost the profits of the pharmaceutical industry.

[125] U.S. airlines had made no major changes as of the beginning of June 2009, but continued standing practices which include looking for passengers with symptoms of flu, measles or other infections, and relying on in-flight air filters to ensure that aircraft were sanitised.

[127][128] U.S. government officials were especially concerned about schools because the H1N1 flu virus appeared to disproportionately affect young and school-age people, between six months and 24 years of age.

[130] The CDC also recommended that colleges should consider suspending fall 2009 classes if the virus began to cause severe illness in a significantly larger share of students than the previous spring.

The guidance suggested that employers consider and communicate their objectives, such as reducing transmission among staff, protecting people who are at increased risk of influenza-related complications from becoming infected, maintaining business operations, and minimising adverse effects on other entities in their supply chains.

[141] Yukihiro Nishiyama, professor of virology at Nagoya University's School of Medicine, commented that the masks are "better than nothing, but it's hard to completely block out an airborne virus since it can easily slip through the gaps".

This term is widely used by mass media, though the Paris-based World Organisation for Animal Health as well as industry groups such as the U.S. National Pork Board, the American Meat Institute, and the Canadian Pork Council objected to widespread media use of the name "swine flu" and suggested it should be called "North American flu" instead, while the World Health Organization switched its designation from "swine influenza" to "influenza A (H1N1)" in late April 2009.

[157] Leading health agencies and the United States Secretary of Agriculture have stressed that eating properly cooked pork or other food products derived from pigs will not cause flu.

The CDC warned that the indiscriminate use of antiviral medications to prevent and treat influenza could ease the way for drug-resistant strains to emerge, which would make the fight against the pandemic that much harder.

[170] Both medications mentioned above for treatment, oseltamivir and zanamivir, have known side effects, including lightheadedness, chills, nausea, vomiting, loss of appetite, and trouble breathing.

They reported that an analysis of 20 studies showed oseltamivir offered mild benefits for healthy adults if taken within 24 hours of onset of symptoms, but found no clear evidence it prevented lower respiratory tract infections or other complications of influenza.

[236] The level of preparedness was further increased and sustained with the advent of the H5N1 bird flu outbreaks because of H5N1's high fatality rate, although the strains currently prevalent have limited human-to-human transmission (anthroponotic) capability, or epidemicity.